[ARM] Set __ARM_FEATURE_SIMD32 for +dsp cores

ARM_FEATURE_DSP is already set for targets with the +dsp feature. In
the backend, this target feature is also used to represent the
availability of the of the instructions that the ACLE guard through
the __ARM_FEATURE_SIMD32 macro. We don't have any cores that
implement one and not the other, so set this macro for cores later
than V6 or for Cortex-M cores that the target parser, or user, reports
that the 'dsp' instructions are supported.

Differential Revision: https://reviews.llvm.org/D51093

llvm-svn: 340911