chacha20poly1305: add runtime internal independent cpu feature detection

Change-Id: I150c5e0453b0fa3457d4786fe90901a54e216b02
Reviewed-on: https://go-review.googlesource.com/41862
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
2 files changed