[libc++abi] Always enable -fvisibility=hidden (#207333)

Building with `-fvisibility=hidden` is generally a good idea, since it
avoids unnecessary relocations when loading a dylib. For libc++abi this
should be a no-op, since most things are annotated explicitly. By
enabling `-fvisibility=hidden` we can remove these explicit annotations
however.

GitOrigin-RevId: cb942d009a5ed029ce20d9b75b3604f551f0cb56