Since libc++abi properly controls what symbols it exports, it is safe
to just re-export the whole library from libc++. This removes some
coupling between libc++ and libc++abi, and reduces the maintenance
burden of having to maintain a list of symbols in libc++.
As part of this change, the ABI list of symbols exported from libc++ is
updated and does not explicitly contain symbols from libc++abi anymore.
This is okay because those symbols are still reachable for any application
linking against libc++, through the whole-dylib re-export.
What's up with the filesystem changes?