Fix problem with moving Xcode location after compiling xctool

xctool depends on the CoreSimulator.framework from Xcode 6 and links
with it using @rpath, which is set to a path withing $(DEVELOPER_DIR)
at compile time.

Then, if Xcode.app is moved after xctool was built, it will fail to find
the framework. I extended the fix for iPhoneSimulatorRemoteClient
introduced in 8f7bf6c64cb64c274649592335eb3d1ee1c850fb to dynamically
update the DYLD_FALLBACK_FRAMEWORK_PATH at runtime to allow finding
CoreSimulator and thus decoupling the binary from the Xcode.app location.