test: add testcase for gccgo-specific issue 10284

Change-Id: I624b336a9eb27fbbc8ef13f141023b4f60966245
Reviewed-on: https://go-review.googlesource.com/8240Reviewed-by: Ian Lance Taylor <iant@golang.org>

test/fixedbugs/issue10284.go

+// compile
+
+// 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 10284: gccgo failed to allow converting a user-defined
+// type whose underlying type is uintptr to unsafe.Pointer.
+
+package p
+
+import "unsafe"
+
+type T uintptr
+
+var _ unsafe.Pointer = unsafe.Pointer(T(0))