libc++ testing: allow to provide a path for `use_system_cxx_lib`

As we're trying to setup testing / bots for all shipping version of libc++
on macOS/iOS, we'll need to be able to pass a path to where to find the
dylib for each previous version of the OS.

Differential Revision: https://reviews.llvm.org/D31486

llvm-svn: 299053