util/u_atomic: Provide a _InterlockedCompareExchange8 for older MSVC.
util/u_atomic: Provide a _InterlockedCompareExchange8 for older MSVC.

Fixes build with Windows SDK 7.0.7600.

Tested with u_atomic_test, both on x86 and x86_64.

Reviewed-by: Roland Scheidegger <7d58aee419d6f9201d75517c885374dff5e7d848@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler