Fix check for extension

Calling 'extension_enabled' this early during the constructor of
'processor_t' causes SIGSEGV.

riscv/processor.cc

@@ -43,7 +43,7 @@ processor_t::processor_t(const isa_parser_t *isa, const cfg_t *cfg,
   TM.proc = this;
 
 #ifndef HAVE_INT128
-  if (extension_enabled('V')) {
+  if (isa->extension_enabled('V')) {
     fprintf(stderr, "V extension is not supported on platforms without __int128 type\n");
     abort();
   }