[CodeGen] Add thumb-mode to target-features for arm/thumb triples.

Summary:
The thumb-mode target feature is used to force Thumb or ARM code
generation on a per-function basis. Explicitly adding +thumb-mode to
functions for thumbxx triples enables mixed ARM/Thumb code generation in
places where compilation units with thumbxx and armxx triples are merged
together (e.g. the IR linker or LTO).

For armxx triples, -thumb-mode is added in a similar fashion.

Reviewers: echristo, t.p.northover, kristof.beyls, rengolin

Reviewed By: echristo

Subscribers: rinon, aemerson, mehdi_amini, javed.absar, cfe-commits

Differential Revision: https://reviews.llvm.org/D33448

llvm-svn: 304897