The iOS simulator compiles with GOOS=darwin GOARCH=386, and x509
sets the inappropriate flag -mmacosx-version-min=10.6. Condition
its compilation on the absence of an "ios" build tag.

Fixes #11736.

Change-Id: I4aa230643347320c3cb9d03b972734b2e0db930e
Reviewed-on: https://go-review.googlesource.com/12301
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>