KWSpec: Cast objc_msgSend for 64-bit device.

Fixes issue #479. See Apple's 64-bit Transition Guide for details:

https://developer.apple.com/library/ios/documentation/General/Conceptual/CocoaTouch64BitGuide/ConvertingYourAppto64-Bit/ConvertingYourAppto64-Bit.html

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Confirmed initial bug and tested the fix on an iPhone 5s.