[libcxx][CI][AIX] Switch to LLVM_ENABLE_RUNTIMES

and to the new `runtimes` top level CMakeLists.txt since the old path is now deprecated. This requires a slight adjustment of the libcxxabi CMake, since there are required macro definitions we previously got via the `llvm/CMakeList.txt` path.

Reviewed By: ldionne, #libc, #libc_abi

Differential Revision: https://reviews.llvm.org/D113403