[ARM] Add support for target("arm") and target("thumb").

Summary:
This patch adds support for the target("arm") and target("thumb")
attributes, which can be used to force the compiler to generated ARM or
Thumb code for a function.

In LLVM, ARM or Thumb code generation can be controlled by the
thumb-mode target feature. But GCC already uses target("arm") and
target("thumb"), so we have to substitute "arm" with -thumb-mode and
"thumb" with +thumb-mode.


Reviewers: echristo, pcc, kristof.beyls

Reviewed By: echristo

Subscribers: ahatanak, aemerson, javed.absar, kristof.beyls, cfe-commits

Differential Revision: https://reviews.llvm.org/D33721

llvm-svn: 304781