dtc: Ensure it is built in C++17 mode

Explicitly specify the C++ standard to be used in the Makefile.
This prevents macOS cross-builds from using the default gnu++98 and
fail. This syncs dtc with upstream commit
39a58cfaab7d55c7975ebf905d859ba91a369fa0.

Reviewed by: emaste
Fixes: 29a55fd09b0a ("dtc: Sync with upstream commit 26a0fe5")
Differential Revision: https://reviews.freebsd.org/D42006

dtc: Sync with upstream commit 26a0fe5

- 0206c0f ("Handle top-level /delete-node/ directives.")
- d612a9e ("Remove C++11 standard constrain")
- Remove extra white lines after the $FreeBSD$ tag removal

Reviewed by: kevans (earlier), theraven, emaste
Differential Revision: https://reviews.freebsd.org/D41482

Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Rework CXXSTD setting via r345708

This change allows the user to once again override the C++ standard, restoring
high-level pre-r345708 behavior.

This also unbreaks building lib/ofed/libibnetdisc/Makefile with a non-C++11
capable compiler, e.g., g++ 4.2.1, as the library supported being built with
older C++ standards.

MFC after: 2 weeks
MFC with: r345708
Reviewed by: emaste
Reported by: jbeich
Differential Revision: https://reviews.freebsd.org/D19895 (as part of a larger change)

Standardize `-std=c++* as `CXXSTD`

CXXSTD was added as the C++ analogue to CSTD.

CXXSTD defaults to `-std=c++11` with supporting compilers; `-std=gnu++98`,
otherwise for older versions of g++.

This change standardizes the CXXSTD variable, originally added to
googletest.test.inc.mk as part of r345203.

As part of this effort, convert all `CXXFLAGS+= -std=*` calls to use `CXXSTD`.

Notes:

This value is not sanity checked in bsd.sys.mk, however, given the two
most used C++ compilers on FreeBSD (clang++ and g++) support both modes, it is
likely to work with both toolchains. This method will be refined in the future
to support more variants of C++, as not all versions of clang++ and g++ (for
instance) support C++14, C++17, etc.

Any manual appending of `-std=*` to `CXXFLAGS` should be replaced with CXXSTD.
Example:

Before this commit:
```
CXXFLAGS+= -std=c++14
```

After this commit:
```
CXXSTD= c++14
```

Reviewed by: asomers
Approved by: emaste (mentor)
MFC after: 1 month
MFC with: r345203, r345704, r345705
Relnotes: yes
Tested with: make tinderbox
Differential Revision: https://reviews.freebsd.org/D19732

Revert r345706: the third time will be the charm

When a review is closed via Phabricator it updates the patch attached to the
review. I downloaded the raw patch from Phabricator, applied it, and repeated
my mistake from r345704 by accident mixing content from D19732 and D19738.

For my own personal sanity, I will try not to mix reviews like this in the
future.

MFC after: 1 month
MFC with: r345706
Approved by: emaste (mentor, implicit)

Standardize `-std=c++* as `CXXSTD`

CXXSTD was added as the C++ analogue to CSTD.

CXXSTD defaults to `-std=c++11` with supporting compilers; `-std=gnu++98`,
otherwise for older versions of g++.

This change standardizes the CXXSTD variable, originally added to
googletest.test.inc.mk as part of r345203.

As part of this effort, convert all `CXXFLAGS+= -std=*` calls to use `CXXSTD`.

Notes:

This value is not sanity checked in bsd.sys.mk, however, given the two
most used C++ compilers on FreeBSD (clang++ and g++) support both modes, it is
likely to work with both toolchains. This method will be refined in the future
to support more variants of C++, as not all versions of clang++ and g++ (for
instance) support C++14, C++17, etc.

Any manual appending of `-std=*` to `CXXFLAGS` should be replaced with CXXSTD.
Example:

Before this commit:
```
CXXFLAGS+= -std=c++14
```

After this commit:
```
CXXSTD= c++14
```

Reviewed by: asomers
Approved by: emaste (mentor)
MFC after: 1 month
MFC with: r345203, r345704, r345705
Relnotes: yes
Tested with: make tinderbox
Differential Revision: https://reviews.freebsd.org/D19732

Revert r345704

I accidentally committed code from two reviews. I will reintroduce the code to
bsd.progs.mk as part of a separate commit from r345704.

Approved by: emaste (mentor, implicit)
MFC after: 2 months
MFC with: r345704

CXXSTD is the C++ analogue to CSTD.

CXXSTD defaults to `-std=c++11` with supporting compilers; `-std=gnu++98`,
otherwise for older versions of g++.

This change standardizes the CXXSTD variable, originally added to
googletest.test.inc.mk as part of r345203.

As part of this effort, convert all `CXXFLAGS+= -std=*` calls to use `CXXSTD`.

Notes:

This value is not sanity checked in bsd.sys.mk, however, given the two
most used C++ compilers on FreeBSD (clang++ and g++) support both modes, it is
likely to work with both toolchains. This method will be refined in the future
to support more variants of C++, as not all versions of clang++ and g++ (for
instance) support C++14, C++17, etc.

Any manual appending of `-std=*` to `CXXFLAGS` should be replaced with CXXSTD.
Example:

Before this commit:
```
CXXFLAGS+= -std=c++14
```

After this commit:
```
CXXSTD= c++14
```

Reviewed by: asomers
Approved by: emaste (mentor)
MFC after: 1 month
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D19732

dtc(1): Revert WARNS change from r328173

WARNS > 3 breaks xtoolchain builds to varying degrees. Revert it.

Reported by: lwhsu, jhb
MFC after: 3 days

dtc(1): Update to upstream ea3c233

Highlights of this update:
- /__local_fixups__ is now generated to be GPL dtc and libfdt compliant
- Compiling with -@ will now cause dtc to assign phandles to all labelled
nodes
- /include/ and /incbin/ now handle absolute paths correctly
- The manpage now has information about overlays, including how to apply
them and how to generate them
- Syntactic sugar for overlays is now supported, allowing an overlay DTS
like:

=
/dts-v1/;
/plugin/;

&foo {
foo,status = "okay";
};
=

to generate a fragment targetting <&foo>.

Improvements to BSD-licensed DTC.

- Added an expression parser so that expressions from headers are now working
- Fixed missing null terminators on cross references
- Disabled exceptions / RTTI in the build for smaller binaries
- Changed phandle order generation to be identical to GPL'd dtc

Ensure that dtc is built in C++11 mode.

Reported by: George Abdelmalik

Import new (BSDL) device tree compiler. Now built by default, so that it can't
be used on the host system (and not installed on the device, if required). The
GPL'd one is still available if there are any devices that need it (make
universe passes with it, including kernels that use fdt, but there may be some
out-of-tree ones). WITH_GPL_DTC can be used to select the old one, for now.

Probably won't be MFC'd, but we'll remove the GPL'd version in head after the
new one has had a lot more testing and ship it in 10.0.