[COFF] Add support for creating range extension thunks for ARM

This is a feature that MS link.exe lacks; it currently errors out on
such relocations, just like lld did before.

This allows linking clang.exe for ARM - practically, any image over
16 MB will likely run into the issue.

Differential Revision: https://reviews.llvm.org/D52156

llvm-svn: 342962