clean up x86_64 (and x32) atomics for new atomics framework

clean up x86_64 (and x32) atomics for new atomics framework

this commit mostly makes consistent things like spacing, function
ordering in atomic_arch.h, argument names, use of volatile, etc.
a_ctz_l was also removed from x86_64 since atomic.h provides it
automatically using a_ctz_64.