[libc++] Fix the runtimes build after making __config_site mandatory

The runtimes build includes libcxx/include/CMakeLists.txt directly instead
of going through the top-level CMake file. This not-very-hygienic inclusion
caused some variables like LIBCXX_BINARY_DIR not to be defined properly,
and the config_site generation logic to fail after landing 53623d4a.

This patch works around this issue by defining the missing variables.
However, the proper fix for this would be for the runtimes build to
always go through libc++'s top-level CMakeLists.txt. Doing otherwise
is unsupported.