In Xcode 6.1 , I am getting error for iPhone 6, iPhone 5s(iOS 7.1) which says
Undefined symbols for architecture x86_64: \"_OBJC_CLASS_$_ClientAut
Here's a response to your latest question about the difference between x86_64 and arm64:
x86_64
arm64
x86_64 architecture is required for running the 64bit simulator.
arm64 architecture is required for running the 64bit device (iPhone 5s, iPhone 6, iPhone 6 Plus, iPad Air, iPad mini with Retina display).