Update the x86 feature-detection code, unifying x86 and x86-64

Update the x86 feature-detection code, unifying x86 and x86-64

Most of it is the same for both of them, so let's avoid doing
everything twice. Or more, since we may support x32 soon. For Windows,
use the intrinsics. For GCC, we'd like to use cpuid.h, but it only
exists since GCC 4.3, so we can't.

And properly detect AVX support: it's not enough to detect that the
processor supports them, we also need to check that the OS enabled
support for the 256-bit registers.

Change-Id: Ibb4872cdb774de5701b18c40f4e612330a266214
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>