[PowerPC] Fix build-id note on powerpc64 kernel

Due to the ordering of the powerpc64 linker script, we were discarding
all notes before emitting .note.gnu.build-id. This had the effect of
generating an empty build id section and breaking the kern.build_id
sysctl added in r348611.

powerpc and powerpcspe are uneffected.

PR: 246430
MFC after: 3 days
Sponsored by: Tag1 Consulting, Inc.