Add size_t, ptrdiff_t, and [u]intptr_t types.

tests/intptr.ispc

@@ -0,0 +1,19 @@
+
+export uniform int width() { return programCount; }
+
+
+export void f_v(uniform float RET[]) {
+    RET[programIndex] = sizeof(uniform intptr_t);
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 
+#if (ISPC_POINTER_SIZE==32)
+        4
+#elif (ISPC_POINTER_SIZE==64)
+        8
+#else
+#error Unknown pointer size
+#endif
+        ;
+}