iOS 8 Compatibility + build script fixes.

Based on the newest changes in WebKit, these changes allow for compiling JavaScriptCore with the latest iOS 8 SDK. Implements the solution described here:
https://bugs.webkit.org/show_bug.cgi?id=129370
http://web.archiveorange.com/archive/v/hdn0HarvfsyR4ZovZly0

Also added a x86_64 slice to make.py/xcodebuild.py for compatibility with 64bit simulators like many of the ones introduced in iOS 8