MFC r364753: Add atomic and bswap functions to libcompiler_rt

There have been several mentions on our mailing lists about missing
atomic functions in our system libraries (e.g. atomic_load_8 and
friends), and recently I saw bswapdi2 and __bswapsi2 mentioned too.

To address this, add implementations for the functions from compiler-rt
to the system compiler support libraries, e.g. libcompiler_rt.a and and
libgcc_s.so.

This also needs a small fixup in compiler-rt's atomic.c, to ensure that
32-bit mips can build correctly.

Bump __FreeBSD_version to make it easier for port maintainers to detect
when these functions were added.

Differential Revision: https://reviews.freebsd.org/D26159

MFC r364782:

After r364753, there should be no need to suppress -Watomic-alignment
warnings anymore for compiler-rt's atomic.c. This occurred because the
IS_LOCK_FREE_8 macro was not correctly defined to 0 for mips, and this
caused the compiler to emit a runtime call to __atomic_is_lock_free(),
and that triggers the warning.

MFC r365509:

Follow-up r364753 by enabling compiler-rt's atomic implementation only
for clang, as it uses clang specific builtins, and does not compile
correctly with gcc. Note that gcc packages usually come with their own
libatomic, providing these primitives.

MFC r365588:

Follow-up r364753 by only using arm's stdatomic.c implementation, as it
already covers the functions in compiler-rt's atomic.c, leading to
conflicts when linking.

PR: 230888
(cherry picked from commit 7c73d99e6cf7b69630856acac4a2ab82f5c9f218)

Approved by: so