As requested during review of D57601, be equally conservative for atomic MMOs as for volatile MMOs in all in tree backends. At the moment, all atomic MMOs are also volatile, but I'm about to change that.
I don't really like this patch since I can't test it. I have no idea what "correct" codegen looks like for these targets, so I'm taking "no tests fail" to mean the patch is correct. If any target owner wants to take over their part and write better tests, please feel free.