Fix building of several libclang_rt libraries for powerpc64 and powerp64le

I reorganized the libclang_rt Makefile in e77a1bb27574 to make it more
readable and maintainable, but the check for 32-bit powerpc was wrong.
This caused almost no libclang_rt libraries to be built for powerpc64
and powerpc64le.

PR: 262706
Reported by: tuexen
Fixes: e77a1bb27574
MFC after: 3 days

(cherry picked from commit f0620ceeccf070a69352105c5dbc23cff499a732)