Add support for sub-architecture when building for iOS

Since iOS 7, the iPhone 5s, the iPad Air and the iPad Mini Retina
supports 64-bits binaries. This requires building a fat binary with
support for both armv7 and arm64 (and optionally armv7s).

In an effort to prepare building such a fat binary for Chrome on
iOS, introduce a "target_subarch" variable that control the variant
we want to build (32-bit only, 64-bit only -- mainly for debugging,
fat binary).

This require introducing way to use the clang compiler shipped with
Xcode, as the clang compiler shipped with Chromium code does not
support "arm64" architecture yet (Apple has not yet opensource the
implementation).

BUG=341453, 314720
R=mark@chromium.org

Review URL: https://codereview.chromium.org/156473005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251083 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251083