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.

PR: 230888
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.