Add an "offset_" prefix to all cpu feature variable offset constants to
signify that they are not boolean cpu feature variables.

Remove _ from offset constant names.

Change-Id: I6e22a79ebcbe6e2ae54c4ac8764f9260bb3223ff
Reviewed-on: https://go-review.googlesource.com/131215
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>