[ARM] Handle +t2dsp feature as an ArchExtKind in ARMTargetParser.def

Currently, the availability of DSP instructions (ACLE 6.4.7) is handled in a
hand-rolled tricky condition block in tools/clang/lib/Basic/Targets.cpp, with
a FIXME: attached.

This patch changes the handling of +t2dsp to be in line with other
architecture extensions.

Following a revert of r248152 and new review comments, this patch also includes
renaming FeatureDSPThumb2 -> FeatureDSP, hasThumb2DSP() -> hasDSP(), etc.
The spelling of "t2dsp" is preserved, pending a further investigation of its
possible external usage.

Differential Revision: http://reviews.llvm.org/D12937

llvm-svn: 248519