[libc++/abi] Revert "[libc++] Move the weak symbols list to libc++abi"

This reverts commit c7d4aa711a. I am still investigating the issue,
but it looks like that commit has an interaction with ld64 that causes
new/delete weak re-exports not to work properly anymore. This is weird
because this commit did not touch the exports of new/delete -- I am
still investigating.

GitOrigin-RevId: d5a6da84a3462d41be003f4e5d4d1d2e0364ce9d
2 files changed