[libc++abi] Fix the standalone build after the __config_site change

In 5d796645, we stopped looking at the LIBCXXABI_LIBCXX_INCLUDES variable,
which broke users of the Standalone build. This patch reinstates that
variable, however it must point to the *installed* path of the libc++
headers, not the libc++ headers in the source tree (which has always
been the case, but wasn't enforced before).

If LIBCXXABI_LIBCXX_INCLUDES points to the libc++ headers in the source
tree, the `__config_site` header will fail to be found.