Slightly reorganize libclang_rt Makefile again

Make a separate .elif section for MACHINE_ARCH==powerpc, and subdivide
the MACHINE_CPUARCH values under it. If at some point more sanitizer
libraries become available for powerpc CPU architectures, they can be
added before the "nothing for other powerpc yet" case. Similar for the
MACHINE_ARCH==arm case.

PR: 262706
Fixes: e77a1bb27574
MFC after: 3 days