ARM MachO embedded: fix test without FileCheck line

Well, that's one way to pass a test, I suppose. Unfortunately actually doing
the testing means I didn't pass all I thought (embedded v7a is not supported,
apparently). I'll deal with that with the move to -none-macho rather than
putting heinous hacks in right now.
Fix pr18235.

The cpp backend is not a reasonable fallback for a missing target. It is a
very special backend, so it is reasonable to use it only if explicitly
requested.

While at it, simplify the interface a bit.