I've added use of Helgrind's `ANNOTATE_HAPPENS_BEFORE()` and `ANNOTATE_HAPPENS_AFTER()` macros to the functions in `atomics.c` and now this is Helgrind-clean as

I've added use of Helgrind's `ANNOTATE_HAPPENS_BEFORE()` and `ANNOTATE_HAPPENS_AFTER()` macros to the functions in `atomics.c` and now this is Helgrind-clean as well as TSAN-clean. The only bugs found by TSAN (as noted in another reply) were the one you found and two assert()s that didn't use atomics. Those two asserts could be a big problem if one ran a non-NDEBUG build in production. So now this is TSAN- and Helgrind-clean. Thanks for prompting me to do this extra work!