Shell out to `uname -r` this time, so that the test will compile
even if the platform doesn't have syscall.Sysctl.

Change-Id: I3a19ab5d820bdb94586a97f4507b3837d7040525
Reviewed-on: https://go-review.googlesource.com/2271Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>