amd64: mostly depessimize copystr

• remove a forward branch in the common case
• replace xchg + lodsb/stosb loop with simple movs

A simple test on Intel(R) Core(TM) i7-4600U CPU @ 2.10GH copying
/foo/bar/baz in a loop goes from 295715863 ops/s to 465807408.

Further changes are pending.

Reviewed by: kib
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D17281