amd64: reimplement libc memset and bzero with kernel memset

This is a depessimization, see r334537 for an explanation. Routines
remain significantly slower than they have to be.

bzero was removed from the kernel but remains in libc. Macroify to
accommodate differences to memset (no return value, always setting to 0).

The bzero.S file is left in place due to libc build magic which pulls in
a C variant if a matching .S file is missing.

Reviewed by: kib
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D17355