[libc++abi/unwind] NFC: Normalize how we set target properties

This is a NFC commit to normalize how we set target properties on the
various runtime targets. A follow-up patch is going to add new properties,
and I wanted that follow-up patch to be cleaner.

GitOrigin-RevId: e95cd94f7edf50367d650410b8e534342eb0e5cb
1 file changed