amd64: convert libc bcopy to a C func to avoid future bloat

The function is of limited use and is an almost a direct clone of
memmove/memcpy (with arguments swapped). Introduction of ERMS variants
of string routines would mean avoidable growth of libc.

bcopy will get redefined to a __builtin_memmove later on with this
symbol only left for compatibility.

Reviewed by: kib
Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17539