cmd/dist: detect sse2 even with gccgo

Change-Id: Idfb20bfe130d9a54d9f5aae8eab8a34655d30610
Reviewed-on: https://go-review.googlesource.com/8865Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/util_gccgo.go

@@ -6,4 +6,15 @@
 
 package main
 
-func cansse2() bool { return false }
+/*
+int supports_sse2() {
+#if defined(__i386__) || defined(__x86_64__)
+	return __builtin_cpu_supports("sse2");
+#else
+	return 0;
+#endif
+}
+*/
+import "C"
+
+func cansse2() bool { return C.supports_sse2() != 0 }