Add clock() function to standard library.

Also corrected the declaration of num_cores() to return a
uniform value.

stdlib.ispc

@@ -787,10 +787,14 @@ packed_store_active(uniform int * uniform a, int vals) {
 ///////////////////////////////////////////////////////////////////////////
 // System information
 
-static inline int num_cores() {
+static inline uniform int num_cores() {
     return __num_cores();
 }
 
+static inline uniform int64 clock() {
+    return __clock();
+}
+
 ///////////////////////////////////////////////////////////////////////////
 // Atomics and memory barriers