Follow CL 63090 and change the Makedev function on Darwin and *BSD to
convert to uint64 before shifting/masking. This avoids a potential
overflow.

Change-Id: I5b566329695cc5edcf82f0ff2366033011b0625b
Reviewed-on: https://go-review.googlesource.com/63112Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>