[OpenMP][mingw] Fix build for aarch64 target

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.

Reviewed By: natgla

Differential Revision: https://reviews.llvm.org/D137168