MFC 332906,332907,332976,333679,336053: Expand testing of breakpoints.

332906:
Extend support for ptrace() tests using breakpoints.

- Use a single list of platforms to define HAVE_BREAKPOINT for platforms
that expose a functional breakpoint() inline to userland. Replace
existing lists of platform tests with HAVE_BREAKPOINT instead.
- Add support for advancing PC past a breakpoint inserted via breakpoint()
to support the existing ptrace__PT_CONTINUE_different_thread test on
non-x86 platforms (x86 advances the PC past the breakpoint instruction,
but other platforms do not). This is implemented by defining a new
SKIP_BREAK macro which accepts a pointer to a 'struct reg' as its sole
argument and modifies the contents to advance the PC. The intention is
to use it in between PT_GETREGS and PT_SETREGS.

332907:
Expose breakpoint() to userland from <machine/cpufunc.h> on MIPS.

Enable ptrace() tests using breakpoint on MIPS as well.

332976:
Shorten some recently-added lines that are an extra indent over 80 columns.

333679:
Export a breakpoint() function to userland for riscv.

As a result, enable tests using breakpoint() on riscv.

336053:
Export a breakpoint() function to userland for arm and arm64.

Enable ptrace() tests using breakpoint() on these architectures.

MFC r316732, r316756: Enable Privileged Access Never on arm64.

r316732:
Use the unprivileged variant of the load and store instructions most
places possible in the kernel. This forces these functions to fail if
userspace is unable to access a given memory location, even if it is in
the user memory range.

This will simplify adding Privileged Access Never support later.

r316756:
In ARMv8.1 ARM has added a process state bit to disable access to userspace
from the kernel. Make use of this to restrict accessing userspace to just
the functions that explicitly handle crossing the user kernel boundary.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add support to the arm64 busdma to handle the cache. For now this is
disabled, however when we enable it it will default to assume memory is
not cache-coherent, unless either the tag was created or the parent was
marked as cache-coherent.

Obtained from: ABT Systems Ltd
Relnotes: yes
Sponsored by: The FreeBSD Foundation

Clear exclusive monitors when handling data aborts, the monitors are
in unknown state per spec.

Reviewed by: andrew (previous version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3668

Bring in the start of the arm64 kernel.

This is only the minimum set of files needed to boot in qemu. As such it is
missing a few things.

The bus_dma code is currently only stub functions with a full implementation
from the development tree to follow.

The gic driver has been copied as the interrupt framework is different. It
is expected the two drivers will be merged by the arm intrng project,
however this will need to be imported into the tree and support for arm64
would need to be added.

This includes code developed by myself, SemiHalf, Ed Maste, and Robin
Randhawa from ARM. This has been funded by the FreeBSD Foundation, with
early development by myself in my spare time with assistance from Robin.

Differential Revision: https://reviews.freebsd.org/D2199
Reviewed by: emaste, imp
Relnotes: yes
Sponsored by: The FreeBSD Foundation

Add the start of the arm64 machine headers. This is the subset needed to
start getting userland libraries building.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation