put mpi's atomics back in, but with the return value of add (and therefore sub, inc...

put mpi's atomics back in, but with the return value of add (and therefore sub, inc, and dec) fixed.

the asm read the value from memory into a register, added to it,
and then tried to write it back. after succeeding it doesnt have
to add again before returning.

this splits sub, inc, and dec off from add. sub can use the subf
opcode, and inc and dec can use the addic opcode. explicitely
identify where the modified memory is so we can avoid using "memory"
as a clobber.

ok mpi@