crypto/subtle: unbreak DIT test on openbsd/arm64

crypto/subtle: unbreak DIT test on openbsd/arm64

OpenBSD already enables DIT on arm64 in both kernel and userland.
As such, if DIT is already enabled, do not expect that it can be
disabled.

Change-Id: If7aae2ace40d97109beefa71c03e0c138a0995ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/629995
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Joel Sing <joel@sing.id.au>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>