This patch changes AArch64 + __GNUC__ to use __sync builtins to
implement internal atomic macros just like for Unix, because mingw-w64
is missing some of the intrinsics which the MSVC codepath is using.
Then some remaining intel-only functions are removed from dllexport to
fix linking.
This should fix https://github.com/llvm/llvm-project/issues/56349.
@natgla - What about this change - was this something which was missed/forgotten and/or just not yet upstreamed from your work on openmp on MSVC/arm64?