chacha20poly1305: scope assembly constants

The names of the constant values were previously not scoped to the
package and |andMask| could collide with the variable of the same name
in the AES-GCM assembly.

Change-Id: I9387a6cace71a585ad2fe2afcc8436d44bde0917
Reviewed-on: https://go-review.googlesource.com/31265
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>