Handle relocations to thumb functions when dynamic linking COFF modules

Summary:
This adds the necessary logic to support relocations to thumb functions in the COFF dynamic linker.
The jumps to function addresses are mostly blx, which requires the ISA selection bit when jumping to a thumb function.

Note: I'm determining if the relocation requires the ISA bit when creating the relocation entries and not when resolving the relocation. I have to do that because I need the ObjectFile and the actual Symbol, which are available only when creating the entries. It would require a gross refactor if I do it otherwise, but I'm okay with doing it if you think it's better.

Reviewers: peter.smith, compnerd

Subscribers: rengolin, sas

Differential Revision: https://reviews.llvm.org/D25151

llvm-svn: 284410