Port from Go internal/cpu

Updates golang/go#25185

Change-Id: I8390980e38b61f6c428fafa0665a03952e7b00bb
Reviewed-on: https://go-review.googlesource.com/c/150718
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>