Provide a sensible default architecture when building on iOS SDKs
https://bugs.webkit.org/show_bug.cgi?id=108395

Patch by David Farler <dfarler@apple.com> on 2013-01-31
Reviewed by Anders Carlsson.

* Scripts/webkitdirs.pm:
(determineArchitecture): Anchor SDK regex matches at the beginning.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141534 268f45cc-cd09-0410-ab3c-d52691b4dbfc