_mm_alignr_epi8 to use compiler builtin

_mm_alignr_epi8 to use compiler builtin

_mm_alignr_epi8 uses the __builtin_ia32_palignr128 function for its
implementation. The builtin function has a different signature on
gcc vs llvm. We provide compiler guards to use the appropriate builtin
function.

Signed-off-by: Felipe Huici <felipe.huici@neclab.eu>
Signed-off-by: Sharan Santhanam <sharan.santhanam@neclab.eu>
Reviewed-by: Vlad-Andrei Badoiu <vlad_andrei.badoiu@stud.acs.upb.ro>