libclang_rt: enable on powerpc*

Summary:
Enable on powerpc64 and in lib/libclang_rt/Makefile change
MACHINE_CPUARCH to MACHINE_ARCH because on powerpc64
MACHINE_ARCH==MACHINE_CPUARCH so the 32-bit library overwrites 64-bit
library during installworld.

This patch doesn't enable any other libclang_rt libraries because they
need to be separately ported.

I have verified that games/julius (which fails on powerpc64 elfv2
without this change because of no libclang_rt profiling library) builds.

Test Plan: Ship it, test on powerpc and powerpcspe

Submitted by: pkubaj
Reviewed by: dim, jhibbits
Differential Revision: https://reviews.freebsd.org/D22425
MFC after: 1 month
X-MFC-With: r353358