PowerPC build fix in mono-hwcap-ppc.
Use GCC atomics when USE_GCC_ATOMIC_OPS is defined.

We still prefer Windows.h functions when compiling for Windows, and
MSVC intrinsics for memory barriers when compiling with MSVC.
Enable GCC atomics on x86 and amd64.
Enable GCC atomics on ppc and ppc64.
Enable GCC atomics on arm.
Enable GCC atomics on mips.