[cmake/multilib] Teach libc++'s CMake build to support multilib libdir
suffixes like 'lib64' or 'lib32'.

This support is currently very rhudimentary. We define a variable
LIBCXX_LIBDIR_SUFFIX. In a standalone build of libc++ this can be
directly set as a cached variable to control the multilib suffix used.
When building libc++ within a larger LLVM build, it is hard wired to
whatever LLVM libdir suffix has been selected. If this doesn't work for
someone, just let me know. I'm happy to change it.

This is essentially new functionality for libc++ so I don't expect it to
have any impact for folks until they start setting these variables.
However, I know libc++ is built in a diverse set of environments so just
let me know if this causes you any problems.