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

This reverts commit c7d4aa71. 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.