sim: sh: clean up time(NULL) call

Casting 0 to a pointer via (long *) doesn't work on LLP64 targets:
error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]

It's also unnecessary here.  We can simply pass NULL like every other
bit of code does.

sim/sh/interp.c

@@ -761,7 +761,7 @@ IOMEM (int addr, int write, int value)
 static int
 get_now (void)
 {
-  return time ((long *) 0);
+  return time (NULL);
 }
 
 static int