[OpenMP] libomp: fix non-X86, non-AARCH64 builds

Commit https://reviews.llvm.org/rG7b5254223acbf2ef9cd278070c5a84ab278d7e5f
broke the build for some architectures, because macro KMP_PREFIX_UNDERSCORE
was defined only for x86, x86_64 and aarch64. This patch defines it for other
architectures (as a no-op).

Differential Revision: https://reviews.llvm.org/D92027