[ARM] Predicate UMAAL selection on hasDSP.

UMAAL is a DSP instruction and it is not available on thumbv7m
(Cortex-M3) and thumbv6m (Cortex-M0+1) targets. Also fix wrong
CHECK prefix in longMAC.ll test.

Patch by Vadzim Dambrouski.

Differential Revision: https://reviews.llvm.org/D25890

llvm-svn: 285278