Clean up backends and tests

[lukaszcz]

We should clean up the CI to avoid running too many tests for things we don't use at this stage. Some of the backends should be delegated to a "supported but not actively updated" status, with their tests removed from the CI, but still runnable manually.

Concretely:

• Move the geb, vampir and wasi compilation targets to dev compile (or extra compile?)
• Remove the tests for geb, vampir and wasi from the CI. Add a new test category extra and put these tests there. The extra tests can still be run manually, but to make the CI faster they shouldn't be run automatically in the CI each time.
• Don't compile the WASM runtime if wasi-sdk cannot be found, removing a hard dependency on it. This requires some conditional compilation in the Haskell sources.

[paulcadman]

Paul: Determine the policy on geb/vamp-ir