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

contrib/llvm-project/compiler-rt/lib/builtins/clear_cache.c

@@ -130,7 +130,10 @@ void __clear_cache(void *start, void *end) {
     __asm __volatile("dsb ish");
   }
   __asm __volatile("isb sy");
-#elif defined(__powerpc64__)
+#elif defined(__powerpc__)
+  // Newer CPUs have a bigger line size made of multiple blocks, so the
+  // following value is a minimal common denominator for what used to be
+  // a single block cache line and is therefore inneficient.
   const size_t line_size = 32;
   const size_t len = (uintptr_t)end - (uintptr_t)start;