amd64: add a mild depessimization to rep mov/stos users

Currently all the primitives are waiting for a rewrite, tidy them up in the
meantime.

Vast majority of cases pass sizes which are multiple of 8. Which means the
following rep stosb/movb has nothing to do. Turns out testing first if there
is anything to do is a big win across the board (cpus with and without ERMS,
Intel and AMD) while not pessimizing the case where there is work to do.

Sample results for zeroing 64 bytes (ops/second):
Ryzen Threadripper 1950X 91433212 -> 147265741
Intel(R) Xeon(R) CPU X5675 @ 3.07GHz 90714044 -> 121992888

bzero and bcopy are on their way out and were not modified. Nothing in the
tree uses them.