Loader: Add load offset to powerpc kernel entry point

Summary:
There is logic in ELF loadimage() to relocate kernels, but currently
only type ET_EXEC. PowerPC kernels are ET_DYN, and can be relocated anywhere.
Add the load offset to kernel entry points on this platform.

Reviewed by: imp, ian
Differential Revision: https://reviews.freebsd.org/D21286