[libc++] Attempt to fix the 'runtimes' build

The 'runtimes' build started failing because libc++ stopped using the
in-tree libc++abi when HAVE_CXXABI is set after 61e89737. This commit
tries to bring back the old behavior when HAVE_CXXABI is set in order
to fix CIs.

However, we really need to sit down and discuss what ways of building
libc++ are supported and formalize them, because having the libc++ build
system branch on basically random variables in some CMake cache somewhere
is not a viable path forward.