Ruby links to `objc` for convenience - this should be moved into a native ext
As a convenance to extension developers Ruby on macOS always links to libobjc. This could be better handled with a native ObjC extension so that one could
require 'objc' to add this to the process.
The extension would link to libobjc and provide bindings to
objc_msgSend and friends. By not linking to libobjc we can also avoid linking to libc++abi and all that entails.
This makes ruby on mac more similar to ruby when compiled on other platforms.