[ARM] Define __ARM_FEATURE_DSP macro for CPUs that have DSP instructions

Summary:
This resolves [[ http://llvm.org/bugs/show_bug.cgi?id=17391 | PR17391 ]].

GCC's sources were used as a guide (couldn't find much information in ARM documentation).

Reviewers: doug.gregor, asl

Reviewed By: asl

Subscribers: asl, aemerson, cfe-commits

Differential Revision: http://reviews.llvm.org/D6339