[cmake][lldb][test] Respect LIBCXX_LIBDIR_SUBDIR (#159106)
This code in principle needs to match the one in libc++ but hasn't been updated to account for
LIBCXX_LIBDIR_SUBDIR so tests don't work if LLVM is built with it specified.
Fixes: ed155f3f237a7e3a3e40a84d815225d27e83f0db
