tests/simple_mt_uaf: don't use stdatomic, clang is braindead
tests/simple_mt_uaf: don't use stdatomic, clang is braindead

Clang, as shipped on Debian and Ubuntu and Fedora, fails so hard when
you include stdatomic.h.  It loads the GCC libstd++ headers, which
conflict with its own atomic crap, and then dies.  Just use GCC atomic
builtins, hopefully these are supported better.

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