Simplify yaml2obj a bit.

The COFFParser now contains only a COFFYAML::Object and the string table
(which is recomputed, not serialized).

The structs in COFFParser now all begin with a Header field with what is
actually on the COFF object. The other fields are things that are semantically
part of the struct (relocations in a section for exmaple), but are not actually
represented that way in the object file.
Change makefile comment to refer to libc++ instead of libcpp.