[VE][compiler-rt] Support VE in clear_cache.c

Support SX Aurora VE by __clear_cache() function.  This modification
allows VE to run written data, e.g. clear_cache_test.c under compiler-rt
test.  We still have code alignment problem in enable_execute_stack_test.c,
though.

Reviewed By: simoll

Differential Revision: https://reviews.llvm.org/D92703

compiler-rt/lib/builtins/clear_cache.c

@@ -167,6 +167,8 @@ void __clear_cache(void *start, void *end) {
 #if __APPLE__
   // On Darwin, sys_icache_invalidate() provides this functionality
   sys_icache_invalidate(start, end - start);
+#elif defined(__ve__)
+  __asm__ volatile("fencec 2");
 #else
   compilerrt_abort();
 #endif