Recommit r248154: [ARM] Handle DSP feature as an ArchExtKind

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 the DSP feature 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: 248521