[libcxx] Allow tests to link with static libc++abi/libc++ even if the shared version is present

Summary:
This is essentially D71894, but for libc++.

This is needed for running libc++ tests over SSH.

Reviewers: EricWF, ldionne, phosek, mehdi_amini, mclow.lists, jroelofs, bcraig, #libc

Reviewed By: ldionne, phosek, #libc

Subscribers: mgorny, christof, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D72687