Use intrinsics instead of assembly

* Intrinsics are always preferable to assembly
 - portability
 - performance: compiler knows how to optimize  them better. with this patch SF is 2.5% faster on my machine!
 . aesthetics: GCC's inline assembly syntax is very ugly.

* As far as I know, these intrinsics can be used in all cases (except MSVC/ICC). Including 32-bit, and ARM. DiscoCheck uses those and has been compiled without modifications on all plateforms (x86, x86-64, ARM).

No functional change.