Add support for host32 for DIRDEPS_BUILD

Allow building 32bit libs for host.

Move CFLAGS additions from local.sys.dirdeps.mk (which is too early
and impacts CFLAGS defaults) to local.sys.mk

Reviewed by: stevek
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D41946

Remove $FreeBSD$: one-line sh pattern

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

local.sys.dirdeps.mk: Allow CROSS_TARGET_FLAGS to be overridden

Use ?= when setting CROSS_TARGET_FLAGS so we do not override it
if another file already has set it.

Reviewed by: sjg
Obtained from: Juniper Networks, Inc.

retire SHARED_TOOLCHAIN knob

Toolchain components were historically statically linked. They became
normal dynamically linked executables in commit 6ab18ea64d19. There is
no need to keep a special case build option for the toolchain; users who
want statically linked toolchain (or any other) components can use the
existing NO_SHARED knob.

Reviewed by: dim, sjg
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41266

Move DIRDEPS_BUILD settings to sys.dirdeps.mk

When originally implemented, there was no distinction between
DIRDEPS_BUILD and META_MODE, they were one and the same.

META_MODE however is useful by itself, but since meta.sys.mk
had lots of settings related to DIRDEPS_BUILD its use was limited
to DIRDEPS_BUILD.

Move (most) DIRDEPS_BUILD related items to sys.dirdeps.mk
so that meta.sys.mk can be used for just META_MODE.
There is of course some bluring of the lines, so settings remain
in meta.sys.mk

Add MK_META_ERROR_TARGET to enable the META_MODE .ERROR target
independent of DIRDEPS_BUILD, it copies failed .meta files to ${SB}/error
to make it easier to identify the cause of build failures.

Since sys.dirdeps.mk should be included first, most of
local.meta.sys.mk becomes local.sys.dirdeps.mk
and some other bits need to move to local.sys.dirdeps.env.mk

Also fix dirdeps.mk to not add CURDIR to DIRDEPS when it is SRCTOP.

Reviewed by: bdrewery
Differential Revision: https://reviews.freebsd.org/D40053