[libc++] Link back-deployment tests against the latest libc++ and libc++abi

Instead of linking the tests against a library in some version of the
SDK, always link against the latest library, but still run against the
specified back-deployment target dylib.

This makes more sense since what we're really trying to test is that
the current library can be used to produce binaries that run on some
deployment target -- not that linking against the library in some
previous SDK makes that possible.

This solves an additional issue that when linking against a system dylib,
the -rpath argument given to the tests is ignored because the install_name
of the system library we link against is absolute.

rdar://63241847