Add support for RDRAND in IvyBridge.

The standard library now provides a variety of rdrand() functions
that call out to RDRAND, when available.

Issue #263.

tests/rdrand-1.ispc

@@ -0,0 +1,21 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+#if !defined(ISPC_TARGET_AVX11) && !defined(ISPC_TARGET_AVX2)
+    RET[programIndex] = 1;
+#else
+
+    uniform float r = -1;
+    uniform int count = 0;
+    while (!rdrand(&r)) {
+        ++count;
+    }
+    RET[programIndex] = (r >= 0 && r < 1);
+
+#endif
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1;
+}