gc: improve unsafe.Pointer type-check error messages

Fixes #2627.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5498088

src/cmd/gc/typecheck.c

@@ -79,6 +79,7 @@ static char* _typekind[] = {
 	[TSTRING]	= "string",
 	[TPTR32]	= "pointer",
 	[TPTR64]	= "pointer",
+	[TUNSAFEPTR]	= "unsafe.Pointer",
 	[TSTRUCT]	= "struct",
 	[TINTER]	= "interface",
 	[TCHAN]		= "chan",

test/fixedbugs/bug390.go

+// errchk $G -e $D/$F.go
+
+// Copyright 2011 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 2627 -- unsafe.Pointer type isn't handled nicely in some errors
+
+package main
+
+import "unsafe"
+
+func main() {
+	var x *int
+	_ = unsafe.Pointer(x) - unsafe.Pointer(x) // ERROR "operator - not defined on unsafe.Pointer"
+}