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

[jhb]

Summary

This fixes a regression in r356418 where it would 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.

Test Plan

• make tinderbox

[jhb]

Not sure how my earlier test missed this. :(