[PECOFF] Fix base relocation block alignment

Base relocation block should be aligned on a 32-bit boundary. While the PECOFF
spec mentions only aligning the blocks, and not padding them, link.exe seems
to add an extra IMAGE_REL_I386_ABSOLUTE entry (just a zeroed WORD) in order to
pad the blocks.

Patch by Ron Ofir.

llvm-svn: 190951