amd64: clean up copyin/copyout

amd64: clean up copyin/copyout

- move the PSL.AC comment to the fault handler
- stop testing for zero-sized ops. after several minutes of package
building there were no copyin calls with zero bytes and very few
copyout. the semantic of returning 0 in this case is preserved
- shorten exit paths by clearing %eax earlier
- replace xchg with 3 movs. this is what compilers do. a naive
benchmark on EPYC suggests about 1% increase in thoughput thanks to
this change.
- remove the useless movb %cl,%al from copyout. it looks like a
leftover from many years ago

Reviewed by:	kib
Approved by:	re (gjb)
Differential Revision:	https://reviews.freebsd.org/D17286