powerpc: implement __clear_cache

Merge LLVM commit 81f5c6270cdfcdf80e6296df216b696a7a37c8b5.

This fixes runtime of most notably pcre libraries (currently patched in ports),
and probably also other ports since currently __clear_cache() just calls
SIGABRT on powerpc.

MFC after: 3 days
Reviewed by: dim
Differential Revision: https://reviews.freebsd.org/D34736