[libc++] Segregate back-deployment dylibs into their own subdirectory

Otherwise, specifying (for example) the libc++.dylib from macos10.13
but the libc++abi.dylib from macos10.12 would end up adding library
paths for both the 10.12 and 10.13 dylibs, which would each contain
a copy of both libc++abi.dylib and libc++.dylib. By using a separate
directory for libc++.dylib and libc++abi.dylib, those do not conflict
anymore.

The back-deployment roots were updated to match this change.