Support for multiarch runtimes layout

Support for multiarch runtimes layout

This change adds a support for multiarch style runtimes layout, so in
addition to the existing layout where runtimes get installed to:

lib/clang/$version/lib/$os

Clang now allows runtimes to be installed to:

lib/clang/$version/$target/lib

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.

The use of new layout is enabled by setting the
LLVM_ENABLE_RUNTIME_TARGET_DIR CMake variable and is supported by both
projects and runtimes layouts. The runtimes CMake build has been further
modified to use the new layout when building runtimes for multiple
targets.

Differential Revision: https://reviews.llvm.org/D45604

llvm-svn: 335809