Add a MONO_VERBOSE_HWCAP environment variable.

This causes detected hardware features to be printed to stdout.
configure: Remove obsolete FIXME comment.
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.
PowerPC build fix in mono-hwcap-ppc.
Enable GCC atomics on ppc and ppc64.
Enable GCC atomics on arm.