[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 lib/Basic/Targets.cpp, with a FIXME:
attached.

http://reviews.llvm.org/D12937 moved the handling of +t2dsp over to
ARMTargetParser.def in LLVM, to be in line with other architecture extensions.

This is the corresponding patch to clang, to clear the FIXME: and update
the tests.

Differential Revision: http://reviews.llvm.org/D12938

llvm-svn: 248154