blake2b: fix constant size

Change-Id: Icae0ec427183d1e319f5a28d6372c823c269d898
Reviewed-on: https://go-review.googlesource.com/34252
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

blake2b/blake2b_amd64.s

@@ -20,7 +20,7 @@ GLOBL ·iv2<>(SB), (NOPTR+RODATA), $16
 
 DATA ·iv3<>+0x00(SB)/8, $0x1f83d9abfb41bd6b
 DATA ·iv3<>+0x08(SB)/8, $0x5be0cd19137e2179
-GLOBL ·iv3<>(SB), (NOPTR+RODATA), $32
+GLOBL ·iv3<>(SB), (NOPTR+RODATA), $16
 
 DATA ·c40<>+0x00(SB)/8, $0x0201000706050403
 DATA ·c40<>+0x08(SB)/8, $0x0a09080f0e0d0c0b