Use internal/cpu instead.

Change-Id: I8670440389cbd88951fee61e352c4a10ac7eee6e
Reviewed-on: https://go-review.googlesource.com/102737
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>