blake2b: use internal/cpu to determine AVX and SSE 4 support

CL 106235 removed runtime.support_avx on which blake2b depends to
determine AVX support. Switch to use the corresponding feature flags
from internal/cpu instead. While at it, also use AVX2 and SSE4 (for Go
1.7 and later) from there.

Change-Id: I4f7a27fcfa0c5d0d7a50444c7aa32519b97e90dd
Reviewed-on: https://go-review.googlesource.com/106336
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

blake2b/blake2bAVX2_amd64.s

@@ -748,15 +748,3 @@ noinc:
 
 	MOVQ BP, SP
 	RET
-
-// func supportsAVX2() bool
-TEXT ·supportsAVX2(SB), 4, $0-1
-	MOVQ runtime·support_avx2(SB), AX
-	MOVB AX, ret+0(FP)
-	RET
-
-// func supportsAVX() bool
-TEXT ·supportsAVX(SB), 4, $0-1
-	MOVQ runtime·support_avx(SB), AX
-	MOVB AX, ret+0(FP)
-	RET