Brings in chacha20poly1305 directory from golang.org/x/crypto revision
12e9ca725de4806fbda1610fd95aacad15bd6810, adding:

CL 41862: chacha20poly1305: add runtime internal independent cpu feature detection
CL 39952: add import comment

Change-Id: Ic46ff24b081bc1c66b6317334d33180e33bfd318
Reviewed-on: https://go-review.googlesource.com/42513
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>