crypto/sha1: disable crashing AVX2 optimizations for now

Updates #15617

Change-Id: I2104776f8e789d987b4f2f7f08f2ebe979b747a1
Reviewed-on: https://go-review.googlesource.com/23001
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Minux Ma <minux@golang.org>

src/crypto/sha1/sha1block_amd64.go

@@ -12,7 +12,9 @@ func blockAVX2(dig *digest, p []byte)
 func blockAMD64(dig *digest, p []byte)
 func checkAVX2() bool
 
-var hasAVX2 = checkAVX2()
+// TODO(TocarIP): fix AVX2 crash (golang.org/issue/15617) and
+// then re-enable this:
+var hasAVX2 = false // checkAVX2()
 
 func block(dig *digest, p []byte) {
 	if hasAVX2 && len(p) >= 256 {