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-3.ispc

@@ -0,0 +1,25 @@
+
+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
+
+    int lessHalf = 0, moreHalf = 0;
+    for (uniform int i = 0; i < 1024*1024; ++i) {
+        float r = -1;
+        while (!rdrand(&r))
+            ;
+        if (r < 0.5) ++lessHalf;
+        else ++moreHalf;
+    }
+
+    float r = (double)lessHalf / (double)(lessHalf + moreHalf);
+    RET[programIndex] = (r >= .49 && r < .51);
+#endif
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1;
+}