[libc++] __config cleanup; _LIBCPP_ABI_UNSTABLE should set _LIBCPP_ABI_VERSION

Some `__config` cleanup and `_LIBCPP_ABI_UNSTABLE` should set `_LIBCPP_ABI_VERSION`, since the latest ABI version //is// the unstable ABI.

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D118989