In order to make conversion to string easier, change the Utsname
members from int8 array to byte array.

Also add a test on Linux for Uname and the string conversion of the
Utsname members.

Fixes golang/go#20753

Change-Id: Iec793dfb4a6dd8f3523cf0b6284a6162074f2e9e
Reviewed-on: https://go-review.googlesource.com/74331
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>