[libc++abi] Simplify the logic for finding libc++ from libc++abi

Since we have the monorepo, libc++abi's build requires a sibling checkout
of the libc++ sources. Hence, the logic for finding libc++ can be greatly
simplified.

GitOrigin-RevId: e619e9d5f542e0764362c2b6762ecd31640651cd
2 files changed