[AMDGPU] Set MaxAtomicSizeInBitsSupported. (#75185)

This will result in larger atomic operations getting expanded to
`__atomic_*` libcalls via AtomicExpandPass, which matches what Clang
already does in the frontend.

While AMDGPU currently disables the use of all libcalls, I've changed it
to instead disable all of them _except_ the atomic ones. Those are
already be emitted by the Clang frontend, and enabling them in the
backend allows the same behavior there.