Fix feature test programs for early POWER7 machines

TestPrograms/test_ppc_power7.cxx

@@ -1,14 +1,38 @@
 #include <altivec.h>
+
+// This follows ppc_simd.h. XLC compilers for POWER7 use vec_xlw4 and vec_xstw4.
+// XLC compilers for POWER8 and above use vec_xl and vec_xst. The way to tell
+// the difference is, POWER7 XLC compilers are version 12.x and earlier.
+// The open question is, how to handle early Clang compilers for POWER7.
+// We know the latest Clang compilers support vec_xl and vec_xst.
+
+#if defined(__xlc__) && (__xlc__ < 0x0d00)
+# define __old_xlc__ 1
+#endif
+
+#if defined(__xlC__) && (__xlC__ < 0x0d00)
+# define __old_xlC__ 1
+#endif
+
 int main(int argc, char* argv[])
 {
-#if defined(__xlc__) || defined(__xlC__) || defined(__clang__)
 	__vector unsigned char x;
+	unsigned char res[16];
+
+#if defined(__old_xlc__) || defined(__old_xlC__)
+    x=vec_xlw4(0, (unsigned char*)argv[0]);
+	x=vec_add(x,x);
+	vec_xstw4(x, 0, res);
+#elif defined(__xlc__) || defined(__xlC__) || defined(__clang__)
 	x=vec_xl(0, (unsigned char*)argv[0]);
 	x=vec_add(x,x);
-#else
+	vec_xst(x, 0, res);
-	__vector unsigned char x;
+#elif defined(__GNUC__)
 	x=vec_vsx_ld(0, (unsigned char*)argv[0]);
 	x=vec_add(x,x);
+	vec_vsx_st(x, 0, res);
+#else
+	int XXX[-1];
 #endif
 	return 0;
 }