[compiler-rt] Fix linking a standalone libatomic for MinGW (#74668)

Whenever linking with -nodefaultlibs for a MinGW target, we manually
need to specify a bunch of libraries - listed in ${MINGW_LIBRARIES}; the
same is already done for sanitizers and libunwind/libcxxabi/libcxx.

Practically speaking, linking with -nodefaultlibs but manually passing
the libraries in ${MINGW_LIBRARIES} restores most of the libraries that
are linked by default, except for the potential compiler builtins and
unwind library; i.e. it has essentially the same effect as linking with
"--unwindlib=none -rtlib=none", except that -rtlib doesn't accept such a
value.

When building only compiler-rt/lib/builtins, not all of compiler-rt,
${MINGW_LIBRARIES} is unset - set it manually here for that case. This
matches what is set in
compiler-rt/cmake/config-ix.cmake, except that the builtins (libgcc or
compiler-rt builtins) is omitted; the only use within lib/buitlins is
for the standalone libatomic, which explicitly already links against the
just-built builtins.