runtime: add TestSizeof

Borrowed from cmd/compile, TestSizeof ensures
that the size of important types doesn't change unexpectedly.
It also helps reviewers see the impact of intended changes.

Change-Id: If57955f0c3e66054de3f40c6bba585b88694c7be
Reviewed-on: https://go-review.googlesource.com/99837
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/export_test.go

@@ -444,3 +444,5 @@ func MapNextArenaHint() (start, end uintptr) {
 func GetNextArenaHint() uintptr {
 	return mheap_.arenaHints.addr
 }
+
+type G = g

src/runtime/sizeof_test.go

@@ -0,0 +1,39 @@
+// Copyright 2018 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.
+
+// +build !nacl
+
+package runtime_test
+
+import (
+	"reflect"
+	"runtime"
+	"testing"
+	"unsafe"
+)
+
+// Assert that the size of important structures do not change unexpectedly.
+
+func TestSizeof(t *testing.T) {
+	const _64bit = unsafe.Sizeof(uintptr(0)) == 8
+
+	var tests = []struct {
+		val    interface{} // type as a value
+		_32bit uintptr     // size on 32bit platforms
+		_64bit uintptr     // size on 64bit platforms
+	}{
+		{runtime.G{}, 216, 376}, // g, but exported for testing
+	}
+
+	for _, tt := range tests {
+		want := tt._32bit
+		if _64bit {
+			want = tt._64bit
+		}
+		got := reflect.TypeOf(tt.val).Size()
+		if want != got {
+			t.Errorf("unsafe.Sizeof(%T) = %d, want %d", tt.val, got, want)
+		}
+	}
+}