Determine whether to build clang and its bootstrap tools the same
way. This allows a clang bootstrap to happen, even when WITHOUT_CLANG
is defined. This is a minimal version of a more extensive change which
can be MFC'd more easily. However, we have to also test to see if
we're building clang as not cc, since the bootstrap for that needs
these cross tools and it is easier to build them in just one place.

MFC after: 1 week