target/s390x: check alignment in CDSG in the !CONFIG_ATOMIC128 case

The CDSG instruction requires a 16-byte alignement, as expressed in
the MO_ALIGN_16 passed to helper_atomic_cmpxchgo_be_mmu. In the non
parallel case, use check_alignment to enforce this.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Message-Id: <20170604202034.16615-4-aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>
