gc: fix unsafe.Sizeof

Fixes #1608.
Fixes #1787.

R=ken2
CC=golang-dev
https://golang.org/cl/4530045

src/cmd/gc/unsafe.c

@@ -41,6 +41,7 @@ unsafenmagic(Node *nn)
 		tr = r->type;
 		if(tr == T)
 			goto bad;
+		dowidth(tr);
 		v = tr->width;
 		goto yes;
 	}

test/fixedbugs/bug338.go

+// $G $D/$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 1787.
+
+package main
+
+import "unsafe"
+
+const x = unsafe.Sizeof([8]byte{})
+
+func main() {
+	var b [x]int
+	_ = b
+}
+
+/*
+bug338.go:14: array bound must be non-negative
+*/

test/fixedbugs/bug339.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// 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 1608.
+// Size used to be -1000000000.
+
+package main
+
+import "unsafe"
+
+func main() {
+	var a interface{} = 0
+	size := unsafe.Sizeof(a)
+	if size != 2*unsafe.Sizeof((*int)(nil)) {
+		println("wrong size: ", size)
+	}
+}