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

[kaz7]

Summary

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.

compiler-rt/lib/builtins/clear_cache.c

#elif defined(__riscv) && defined(__linux__)
   __asm __volatile("ecall"
                    : "=r"(start_reg)
                    : "r"(start_reg), "r"(end_reg), "r"(flags), "r"(syscall_nr));
   assert(start_reg == 0 && "Cache flush syscall failed.");
 #else
 #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
 #endif
 }