This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project release/21.x llvmorg-21.1.7-0-gcd708029e0b2,
a.k.a. 21.1.7 release.

PR:		292067
MFC after:	1 month

Fix various configuration fails, update generated headers, Makefiles, etc.

PR:		292067
MFC after:	1 month

Upstream llvm compiles most of their subprojects with -std=c++17
explicitly, not -std=gnu++17. In gnu++17 mode, both clang and gcc define
the macro 'i386' on i386, which clashes with a namespace identifier in
contrib/llvm-project/llvm/include/llvm/ExecutionEngine/JITLink/i386.h.

Since the default after 6527682ab705 is now CXXSTD=gnu++17, explicitly
set CXXSTD=c++17 for lib/clang, lib/libclang_rt and lib/libomp.

Previously the compiler's default C++ standard was used unlike C where
bsd.sys.mk explicitly sets a default language version.  Setting an
explicit default version will give a more uniform experience across
different compilers and compiler versions.

gnu++17 was chosen to match the default C standard.  It is well
supported by a wide range of clang (5+) and GCC (9+) versions.

gnu++17 is also the default C++ standard in recent versions of clang
(16+) and GCC (11+).  As a result, many of the explicit CXXSTD
settings in Makefiles had the effect of lowering the C++ standard
instead of raising it as was originally intended and are removed.

Note that the remaining explicit CXXSTD settings for atf and liblutok
explicitly lower the standard to C++11 due to use of the deprecated
auto_ptr<> template which is removed in later versions.

Reviewed by:	imp, asomers, dim, emaste
Differential Revision:	https://reviews.freebsd.org/D49223

It was disabled by default in fe52b7f60ef4.  We planned to (but did not)
remove the option before FreeBSD 14.  Remove it now, for FreeBSD 15.

Relnotes:	Yes
Reviewed by:	imp
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31558

This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project main llvmorg-19-init-18630-gf2ccf80136a0, the
last commit before the upstream release/19.x branch was created.

PR:		280562
MFC after:	1 month

This is a residual of the $FreeBSD$ removal.

MFC After: 3 days (though I'll just run the command on the branches)
Sponsored by: Netflix

We just need to check we are building for arm.

Reviewed by:	manu, emaste
Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D45644

This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project main llvmorg-18-init-15088-gd14ee76181fb.

PR:		276104
MFC after:	1 month

Make a separate .elif section for MACHINE_ARCH==powerpc, and subdivide
the MACHINE_CPUARCH values under it. If at some point more sanitizer
libraries become available for powerpc CPU architectures, they can be
added before the "nothing for other powerpc yet" case. Similar for the
MACHINE_ARCH==arm case.

PR:		262706
Fixes:		e77a1bb27574
MFC after:	3 days