crypto/x509: iOS build tag

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>

src/crypto/x509/root_cgo_darwin.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo,!arm,!arm64
+// +build cgo,!arm,!arm64,!ios
 
 package x509
 

src/crypto/x509/root_darwin_armx.go

@@ -5,7 +5,7 @@
 // license that can be found in the LICENSE file.
 
 // +build darwin
-// +build arm arm64
+// +build arm arm64 ios
 
 package x509