amd64: replace libkern's memset and memmove with assembly variants

memmove is repurposed bcopy (arguments swapped, return value added)
The libkern variant is a wrapper around bcopy, so this is a big
improvement.

memset is repurposed memcpy. The librkern variant is doing fishy stuff,
including branching on 0 and calling bzero.

Both functions are rather crude and subject to partial depessimization.

This is a soft prerequisite to adding variants utilizing the
'Enhanced REP MOVSB/STOSB' bit and let the kernel patch at runtime.