Allow running back-deployment testing against libc++abi

Summary:
Before this patch, we could only link against the back-deployment libc++abi
dylib. This patch allows linking against the just-built libc++abi, but
running against the back-deployment one -- just like we do for libc++.

Also, add XFAIL markup to flag expected errors.

GitOrigin-RevId: 4d79ef814aedb63e85903bb145dfe7ec143c6909
17 files changed