[AArch64][RISCV] Make sure isel correctly honors failure orderings.

If a cmpxchg specifies acquire or seq_cst on failure, make sure we
generate code consistent with that ordering even if the success ordering
is not acquire/seq_cst.

At one point, it was ambiguous whether this sort of construct was valid,
but the C++ standad and LLVM now accept arbitrary combinations of
success/failure orderings.

This doesn't address the corresponding issue in AtomicExpand. (This was
reported as https://bugs.llvm.org/show_bug.cgi?id=33332 .)

Fixes https://bugs.llvm.org/show_bug.cgi?id=50512.

Differential Revision: https://reviews.llvm.org/D103284