amd64: clear PSL.AC in the right frame

If copyin family of routines fault, kernel does clear PSL.AC on the
fault entry, but the AC flag of the faulted frame is kept intact. Since
onfault handler is effectively jump, AC survives until syscall exit.

Reported by: m00nbsd, via Sony
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
admbugs: 975

(cherry picked from commit 91aae953cb807d6fb7a70782b323bf9beb60d7c9)