Limit Options.inc generation to desired targets

As mentioned in the Makefile there's an "atrocious" hack to generate a
different version of Options.inc.h, depending on the library being
built.

Remove the catch-all else case and limit it to specific libraries, so
that we don't accidentally use the Options.inc.h from clangdriver if a
future libary also uses Options.inc.h.

Reviewed by: dim
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6209