Detect CPU features at runtime via cpuid - code borrowed from
libavutil.
Availability of cpuid is not checked since pentium3 supports it
and that was the minimum requirement anyway.
Only features enabled at compile time will be available.
Forced MMX/SSE/SSE2/SSE3 and it passed all tests via:
./doom3.x86_64 +disconnect +set s_noSound 1 +testSIMD
Thread return types are different between platforms, and its
probably not a good idea to return something of a different size,
cast the callback and expect it to not crash.
