crypto/x509: generate new-style build tags for iOS

Make the input match gofmt's output,
to make our lives easier as we phase out
old style build tags.

Change-Id: I95dc5a77058bf17cb02e289703f60784616db006
Reviewed-on: https://go-review.googlesource.com/c/go/+/358934


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>