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