x86-64: Optimize bzero

memset with zero as the value to set is by far the majority value (99%+
for Python3 and GCC).

bzero can be slightly more optimized for this case by using a zero-idiom
xor for broadcasting the set value to a register (vector or GPR).

Co-developed-by: Noah Goldstein <goldstein.w.n@gmail.com>
(cherry picked from commit 3d9f171b)