Enable 64-bit architectures by default for Xcode 5.1 and higher

According to Xcode 5.1 release notes, the "Standard Architectures" build
settings now also include 64-bit architecture, while the build settings
"Standard Architectures Including 64-Bit" is no deprecated.

BUG=gyp:412
R=mark@chromium.org

Review URL: https://codereview.chromium.org/194713003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1882 78cadc50-ecff-11dd-a971-7dbc132099af