amd64: clean up copyin/copyout

[mjg]

Summary

• 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 uselss movb %cl,%al from copyout. it looks like a leftover from many years ago

[kib]

movb is removed from copyin.

I am curious about the removal of xchg reg,reg.