Make rn_xplat_cxx_library and rn_apple_library target iOS 11
Make rn_xplat_cxx_library and rn_apple_library target iOS 11

Summary:
This forces individual RN libraries to target SDK 11+

Changelog: [iOS] Make rn_xplat_cxx_library and rn_apple_library target iOS 11

Reviewed By: shergin

Differential Revision: D26179411

fbshipit-source-id: 898201943164ff8e53924eee5ca877103bf0f62a

janicduplessis/react-native,javache/react-native,facebook/react-native,myntra/react-native,janicduplessis/react-native,facebook/react-native,javache/react-native,facebook/react-native,arthuralee/react-native,javache/react-native,pandiaraj44/react-native,facebook/react-native,myntra/react-native,arthuralee/react-native,facebook/react-native,pandiaraj44/react-native,myntra/react-native,pandiaraj44/react-native,facebook/react-native,janicduplessis/react-native,pandiaraj44/react-native,facebook/react-native,janicduplessis/react-native,arthuralee/react-native,arthuralee/react-native,myntra/react-native,myntra/react-native,javache/react-native,arthuralee/react-native,pandiaraj44/react-native,janicduplessis/react-native,javache/react-native,myntra/react-native,janicduplessis/react-native,myntra/react-native,pandiaraj44/react-native,javache/react-native,facebook/react-native,janicduplessis/react-native,myntra/react-native,pandiaraj44/react-native,javache/react-native,pandiaraj44/react-native,myntra/react-native,janicduplessis/react-native,facebook/react-native,javache/react-native,javache/react-native