[Sparc] limit MaxAtomicSizeInBitsSupported to 32 for 32-bit Sparc. (#81655)

When in 32-bit mode, the backend doesn't currently implement 64-bit
atomics, even though the hardware is capable if you have specified a V9
CPU. Thus, limit the width to 32-bit, for now, leaving behind a TODO.

This fixes a regression triggered by PR #73176.