[COFF] Add support for aligncomm directives

These are emitted for comm symbols in object files, when targeting
a GNU environment.

Alternatively, just ignore them since we already align CommonChunk
to the natural size of the content (up to 32 bytes). That would only
trade away the possibility to overalign small symbols, which doesn't
sound like something that might not need to be handled?

Differential Revision: https://reviews.llvm.org/D36304

llvm-svn: 310871