Originally we disabled libcompiler-rt on MIPS and SPARC64, because of an
issue where __clzdi2 and __ctzdi2 would cause endless recursion. This
bug has been fixed in r230021 already, but for some reason we only
switched to libcompiler-rt on SPARC64 -- not MIPS.
This means we can finally use <stdatomic.h> on all our architectures.