[PECOFF] Implement Win64 base relocations

Image Base field in the PE/COFF header is used as hint for the loader.
If the loader can load the executable at the specified address, that's
fine, but if not, it has to load it at a different address.

If that happens, the loader has to fix up the addresses in the
executable by adding the offset. The list of addresses that need to
be fixed is in .reloc section.

This patch is to emit x64 .reloc section contents.

llvm-svn: 216636