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>
diff --git a/blake2s/blake2s_amd64.go b/blake2s/blake2s_amd64.go
index 6c5d830..c88f330 100644
--- a/blake2s/blake2s_amd64.go
+++ b/blake2s/blake2s_amd64.go
@@ -7,8 +7,10 @@
 package blake2s
 
 var (
-	useSSE4  = supportSSE4()
-	useSSSE3 = supportSSSE3()
+	useSSE4    = supportSSE4()
+	useSSSE3   = supportSSSE3()
+	useSSE2    = true // Always available on amd64
+	useGeneric = false
 )
 
 //go:noescape