[MC, COFF] Add .reloc support for WinCOFF

This adds rudimentary support for a few relocations that we will use for
the CodeView debug format.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@258216 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed