[libc++abi] Remove XFAIL on arm64

The underlying TLS destruction order bug has been fixed in the OS. This
would technically still fail when running on top of macOS < 12, however
we don't have a good way of encoding that using Lit features. Indeed,
the existing target=<FOO> Lit feature encodes the deployment target,
not the actual runtime system that the tests are being run on.

If this test starts failing on your machine after this patch, upgrading
to macOS 12 should solve the problem.