Fix ARCHS selection when building for iOS

When building for iOS, consider any architecture set in the gyp files
as valid (to allow building for x86_64 when using Xcode 4.0 or higher).

BUG=webrtc:3073
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/203763002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1881 78cadc50-ecff-11dd-a971-7dbc132099af