target/s390x: Use cpu_{ld,st}*_mmu in do_csst

target/s390x: Use cpu_{ld,st}*_mmu in do_csst

Use cpu_ld16_mmu and cpu_st16_mmu to eliminate the special case,
and change all of the *_data_ra functions to match.

Note that we check the alignment of both compare and store
pointers at the top of the function, so MO_ALIGN* may be
safely removed from the individual memory operations.

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