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 libkbo

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.

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