Avoid using package specific variables when there is a one to one
correspondance to cpu feature support exported by internal/cpu.

This makes it clearer which cpu feature is referenced.
Another advantage is that internal/cpu variables are padded to avoid
false sharing and memory and cache usage is shared by multiple packages.

Change-Id: Ieadfc2f2f65f83f947aa8a5efc869aa85d89615d
Reviewed-on: https://go-review.googlesource.com/126597
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>