Fix python warning in syscall stub generator

The "is" keyword compares addresses. "==" compares values.

libsel4/tools/syscall_stub_gen.py

@@ -1077,7 +1077,7 @@ def main():
     else:
         wordsize = int(args.wsize)
 
-    if wordsize is -1:
+    if wordsize == -1:
         print("Invalid word size.")
         sys.exit(2)