Check the size of uintptr instead of listing GOARCHes explicitly. This
will make the test also run on linux/mips{,le}.

Change-Id: I649f15d293002afc1360b1913910202c3e5188b7
Reviewed-on: https://go-review.googlesource.com/135715
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>