This change adds a support for multiarch style runtimes layout, so in
addition to the existing layout where runtimes get installed to:
Clang now allows runtimes to be installed to:
This also includes libc++, libc++abi and libunwind; today those are
assumed to be in Clang library directory built for host, with the
new layout it is possible to install libc++, libc++abi and libunwind
into the runtime directory built for different targets.
Currently the new layout is support for Fuchsia and Linux.
The runtimes CMake build has been modified to use the new layout when
building runtimes for multiple targets.