api: add exception for os.ModeType value

The ModeType constant is a bit-wise OR of several other values.
It is reasonable for this to change without breaking compatibility.

This fixes the failures introduced by https://golang.org/cl/103597

Change-Id: I817e351cf3d1e0115dce665c8efbce21bc36426c
Reviewed-on: https://go-review.googlesource.com/103616
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

api/except.txt

@@ -2,6 +2,7 @@ pkg encoding/json, method (*RawMessage) MarshalJSON() ([]uint8, error)
 pkg math/big, const MaxBase = 36
 pkg math/big, type Word uintptr
 pkg net, func ListenUnixgram(string, *UnixAddr) (*UDPConn, error)
+pkg os, const ModeType = 2399141888
 pkg os (linux-arm), const O_SYNC = 4096
 pkg os (linux-arm-cgo), const O_SYNC = 4096
 pkg syscall (darwin-386), const ImplementsGetwd = false