compiler-rt uses atomic built-ins such as sync_val_compare_and_swap()
which are not available for 64-bit values on 32-bit SPARC. Since these
built-ins are considered legacy by GCC upstream and have been replaced
by the atomic_* built-ins, there are no plans to implement a 64-bit
version of __sync_val_compare_and_swap() for 32-bit SPARC (PR c/63368).
Thus, disable compiler-rt on 32-bit SPARC for the time being which is
what the build system already does for 32-bit PowerPC where compiler-rt
isn't built either.