additionally force the install step to be non-parallel, as there's
something broken (at least with gcc 4.8.5, some commands get executed
in the parent shell after make already finished).
previously the stage0 gcc was installed to the output location during
the build, then replaced later by the final gcc. this violates the
principles of make and simply wastes time. the stage0 gcc is usable
in-place without installing it, so we can simply do that instead.