util: Cleanup u_cpu_detect, build. Support X86_64 and detect SSE4.1 too.

util: Cleanup u_cpu_detect, build. Support X86_64 and detect SSE4.1 too.

I was waiting for the need to use this code to arise, and it finally came.

I've tested building this on Linux and Windows, both x86 and x64_64. But
it might break other platforms. Please bear with me and help me fix it.

Many thanks to Dennis Smit who submitted this, and Eric Anholt whose
work this was based on.