[libcxx] Allow tests to link with static libc++abi/libc++ even if the shared version is present
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
Differential Revision: https://reviews.llvm.org/D72687