Because you can't bind a thread to the second CPU if it's not there.

Change-Id: I2af4e66667711dc9a3aa23fee073a70cdc5fdd4d
Reviewed-on: https://go-review.googlesource.com/99355
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>