chacha20poly1305: scope assembly constants
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 <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/crypto,ScriptRock/crypto,ScriptRock/crypto,golang/crypto,golang/crypto