Don't compile the x86_32 only memcpy/memcmp functions.

These optimized codepaths only work on x86_32, use the ones provided by the toolchain since it should be good enough.

Re-arranges a #ifdef __LINUX__ in MemcpyFast.h since __LINUX__ is much more restricting than _WIN32.
__LINUX__ isn't defined when building on FreeBSD for example. Not a supported platform, but make easy decisions now.

This requires PR #109 & #128 to be merged first.