powerpc64: Fix kernel ldscript to only emit one PT_LOAD segment

Summary:
kexec-lite cannot currently handle multiple PT_LOAD segments. In some
cases the compiler generates multiple PT_LOAD segments for an unknown
reason, causing boot to fail from kexec-lite.

Submitted by: Brandon Bergren (older version)
Differential Revision: https://reviews.freebsd.org/D19574