kboot: Cleanup libkern reference

For aarch64 and amd64, we don't pull in anything from libkern, so we
don't need it in our path. However, powerpc needs ucmpdi2 from libkern,
so bring it into libkboot's build and omit it from loader.kboot.

Sponsored by: Netflix

(cherry picked from commit 4aabaa1890e927ed3dd34ccc4642268ea6a505ce)