We had a single ATOMIC_MMU_LOOKUP macro that probed for
read+write on all atomic ops.  This is incorrect for
plain atomic load and atomic store.

For user-only, we rely on the host page permissions.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/390


Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>