Changes all cpu features to be detected and stored in bools in rt0_go.

Updates: #15403

Change-Id: I5a9961cdec789b331d09c44d86beb53833d5dc3e
Reviewed-on: https://go-review.googlesource.com/41950
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Keith Randall <khr@golang.org>