[libcxx] [libcxxabi] [libunwind] [llvm] libcxx: enable tests against wasi-libc (PR #215384)

[Nikolas Klauser via cfe-commits]

https://github.com/philnik777 commented:

Just to be clear, the intention of the various `-FIXME` features is that they are eventually removed. A few of the comments seem to imply that there is no intention of supporting the feature. If that's the case, the tests should be disabled via `target=` instead of the `-FIXME` feature. The tests can of course also be modified to test the specific behaviour of wasm if feasible, and in those cases the `-FIXME` should be used.

Looks like we need to update the CMake version in the docker image to allow building the wasi-libc. I'll open a patch, since we need to do that anyways.

https://github.com/llvm/llvm-project/pull/215384