Before making top-level traverse js/src moz.build files, there was a need to
distinguish between top-level traversing e.g. top-level moz.build or
config/moz.build and js/src traversing them. With a single traversal of both
moz.build sets, we now only need to distinguish between js standalone builds
and gecko builds.
There is still, however, a need to distinguish between top-level vs. js/src
configure runs on gecko builds to make them subconfigure icu and libffi from
top-level instead of js/src in js standalone builds, or when choosing to make
js/src's config.status do something when run or not.
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.
