[libc++] Simplify how we re-export symbols from libc++abi

Instead of managing two copies of the symbol lists, reuse the same list
in libc++abi and libc++.

Differential Revision: https://reviews.llvm.org/D88623

GitOrigin-RevId: 4f13b999297140486b2faa1b5d8d7c768fb40dfb
1 file changed