Completed :lower16: / :upper16: support for movw / movt pairs on Darwin.

- Fixed :upper16: fix up routine. It should be shifting down the top 16 bits first.
- Added support for Thumb2 :lower16: and :upper16: fix up.
- Added :upper16: and :lower16: relocation support to mach-o object writer.

llvm-svn: 123424