[multilib] Add support to the autoconf build to substitute
a CLANG_LIBDIR_SUFFIX variable. This is necessary before I can add
support for using that variable to CMake and the C++ code in Clang, and
the autoconf build system does all substitutions in the LLVM tree.

As mentioned before, I'm not planning to add actual multilib support to
the autoconf build, just enough stubs for it to keep playing nicely with
the CMake build once that one has support.