Bug 664362 - Generate depfiles for host objects/programs/libraries. r=gps
Bug 928204 - Don't ignore missing depfiles when aggregating them. r=gps

However, since we don't generate depfiles when compiling assembly, explicitly exclude those.
Also, fix a few issues with included depfiles:
- The tools tier need them because it (sadly) links binaries.
- depfiles for TARGETS use the full path, whereas the depfiles for OBJS and others don't.
Bug 928929 - Kill local rule in testing/mochitest/chrome/Makefile.in. r=gps