libc/atomic: Fix membars in __atomic_load/store_* stubs.

- membar_enter/exit ordering was backwards.
- membar_enter doesn't make any sense for load anyway.
- Switch to membar_release for store and membar_acquire for load.

The only sensible orderings for a simple load or store are acquire or
release, respectively, or sequential consistency.  This never
provided correct sequential consistency before -- we should really
make it conditional on memmodel but I don't know offhand what the
values of memmodel might be and this is at least better than before.

(riastradh)