[libc++] Upstream ptrauth support in libc++ and libc++abi (#84573)

This is an exact upstreaming of the downstream diff. Minor
simplifications can be made in the future but upstreaming as-is will
make it easier for us to deal with downstream merge conflicts.

Partially fixes #83805

GitOrigin-RevId: 98244c4e2acafb7568e8337088c6caaaffcb7831
1 file changed