COFF: Support base relocations.

PE/COFF executables/DLLs usually contain data which is called
base relocations. Base relocations are a list of addresses that
need to be fixed by the loader if load-time relocation is needed.

Base relocations are in .reloc section.

We emit one base relocation entry for each IMAGE_REL_AMD64_ADDR64
relocation.

In order to save disk space, base relocations are grouped by page.
Each group is called a block. A block starts with a 32-bit page
address followed by 16-bit offsets in the page. That is more
efficient representation of addresses than just an array of 32-bit
addresses.

llvm-svn: 239710