Don't fail universe kernel stage for TARGET_ARCHes without a kernel config.

This fixes a regression in r356418 where the entire universe would
fail early due to an undefined make target when a given TARGET_ARCH
had no associated kernel configs. This is true for all of the
hard-float mips TARGET_ARCHes currently.

Pointy hat to: me
Reviewed by: emaste
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D23071