[ARM] Enable SMLAL[B|T] isel

Enable the selection of the 64-bit signed multiply accumulate
instructions which operate on 16-bit operands. These are enabled for
ARMv5TE onwards for ARM and for V6T2 and other DSP enabled Thumb
architectures.

Differential Revision: https://reviews.llvm.org/D30044

llvm-svn: 297809