Change creation of relative relocations on COFF.

For whatever reason, when processing

  .globl foo
foo:
  .data
bar:
  .long foo-bar

llvm-mc creates a relocation with the section:

0x0 IMAGE_REL_I386_REL32 .text

This is different than when the relocation is relative from the
beginning. For example, a file with

call foo

produces

0x0 IMAGE_REL_I386_REL32 foo

I would like to refactor the logic for converting "foo - ." into a
relative relocation so that it is shared with ELF. This is the first
step and just changes the coff implementation to match what ELF (and
COFF in the case of calls) does.

llvm-svn: 306063