obj2yaml, yaml2obj: Add support for COFF executables

In support of serializing executables, obj2yaml now records the virtual address
and size of sections.  It also serializes whatever we strictly need from
the PE header, it expects that it can reconstitute everything else via
inference.

yaml2obj can reconstitute a fully linked executable.

In order to get executables correctly serialized/deserialized, other
bugs were fixed as a circumstance.  We now properly respect file and
section alignments.  We also avoid writing out string tables unless they
are strictly necessary.

llvm-svn: 221975