xcode_emulation: added support for -fobjc-arc

This is needed for projects that want to use ARC
(http://clang.llvm.org/docs/AutomaticReferenceCounting.html)

Note that using ARC has implications on where your binary
can run; see  for details (or be happy with restricting to
OS/X 10.8+ and iOS 6.0+).

BUG=https://code.google.com/p/webrtc/issues/detail?id=2106
TESTED=git try, works in libjingle
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/19708002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1681 78cadc50-ecff-11dd-a971-7dbc132099af