internal/cpu: fix test build on ppc64

The runtime import is unused.

Change-Id: I37fe210256ddafa579d9e6d64f3f0db78581974e
Reviewed-on: https://go-review.googlesource.com/114175
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Austin Clements <austin@google.com>

src/internal/cpu/cpu_ppc64x_test.go

@@ -8,7 +8,6 @@ package cpu_test
 
 import (
 	. "internal/cpu"
-	"runtime"
 	"testing"
 )