blake2s: test all four hashing implementations

This adds use* flags for all arches so a common test can observe what
implementations are supported and test all supported implementations.

Change-Id: Icc9c3c1d15626e95f0446493b7fa3159bbe9567d
Reviewed-on: https://go-review.googlesource.com/31712
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Andreas Auernhammer <aead@mail.de>