Darwin doesn't have a uname syscall but the same information can be
retrieved using sysctls. Use the raw sysctl numbers instead of doing the
roundtrip via SysctlString in order to void the additional sysctl calls
in nametomib and SysctlRaw.

This implementation follows the one in
https://opensource.apple.com/source/Libc/Libc-1244.1.7/gen/uname.c

Change-Id: I19541e2a6700f84ced0549a8fb86faa8cb5e6fbe
Reviewed-on: https://go-review.googlesource.com/77270
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>