[PowerPC] Fix powerpc64le boot after HPT superpages addition

The HPT is always stored in big-endian, as it is accessed directly by the
hardware as well as the kernel. As such, it is necessary to convert values
to and from native endian when running on LE.

Some unconverted accesses snuck in accidentally with r367417.

Apply the appropriate conversions to fix boot hanging on powerpc64le.

Sponsored by: Tag1 Consulting, Inc.