[libc++][modules] Re-add build dir CMakeLists.txt. (#81370)

This CMakeLists.txt is used to build modules without build system
support. This was removed in d06ae33e.
This is used in the documentation how to use modules.

Made some minor changes to make it work with the std.compat module using
the std module.

Note the CMakeLists.txt in the build dir should be removed once build
system support is generally available.