[libc++abi] Assume that libcxx is checked out in the Apple build script

Instead of using the libc++ headers provided alongside the toolchain,
use those in the sibling libcxx directory that we know is checked out.
Before the days of the monorepo, we couldn't assume that the libc++
repository was present when building libcxxabi. Since we can now make
that assumption, it's always better to use the version of libc++ that
is in lockstep with libc++abi, to avoid subtle bugs.