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-4.ispc

@@ -0,0 +1,33 @@
+
+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] = 0;
+#else
+
+    uniform int set[64] = { 0 };
+    uniform int count = 1024*1024;
+    for (uniform int i = 0; i < count; ++i) {
+        uniform int64 r;
+        while (!rdrand(&r))
+            ;
+        for (uniform int b = 0; b < 64; ++b) 
+            if (((unsigned int64)r >> b) & 1)
+                ++set[b];
+    }
+
+    RET[programIndex] = 0;
+    for (uniform int b = 0; b < 64; ++b)  {
+        float r = (double)set[b] / (double)(count);
+        if (!(r >= .49 && r < .51)) {
+            print("% % - %\n", b, r, set[b]);
+            ++RET[programIndex];
+        }
+    }
+#endif
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0;
+}