test: add gccgo test case for #10407

Change-Id: I8d17e2b0fbc529ca7958c75222964a5e419aa3db
Reviewed-on: https://go-review.googlesource.com/8717Reviewed-by: Ian Lance Taylor <iant@golang.org>

test/fixedbugs/issue10407.go

+// runoutput
+
+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 10407: gccgo failed to remove carriage returns
+// from raw string literals.
+
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("package main\nfunc main() { if `a\rb\r\nc` != \"ab\\nc\" { panic(42) }}")
+}