crypto/sha512: fix checkAVX2

The checkAVX2 test doesn't appear to be correct,
because it always returns the value of support_bmi2,
even if the value of support_avx2 is false.

Consequently, checkAVX2 always returns true, as long
as BMI2 is supported, even if AVX2 is not supported.

We change checkAVX2 to return false when support_avx2
is false.

Fixes #19316.

Change-Id: I2ec9dfaa09f4b54c4a03d60efef891b955d60578
Reviewed-on: https://go-review.googlesource.com/37590
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/crypto/sha512/sha512block_amd64.s

@@ -1480,6 +1480,7 @@ TEXT ·checkAVX2(SB), NOSPLIT, $0
 	CMPB AX,$0
 	JNE check_bmi2
 	MOVB AX, ret+0(FP)
+	RET
 check_bmi2:
 	MOVB runtime·support_bmi2(SB), AX
 	MOVB AX, ret+0(FP)