[ARM] Add __ARM_FEATURE_DOTPROD pre-defined macro

This adds a pre-defined macro to test if the compiler has support for the
v8.2-A dot rpoduct intrinsics in AArch32 mode.

The AAcrh64 equivalent has already been added by rL330229.

The ACLE spec which describes this macro hasn't been published yet, but this is
based on the final internal draft, and GCC has already implemented this.

Differential revision: https://reviews.llvm.org/D46108

llvm-svn: 331038