Cranelift: audit runtests for skipped platforms and add support where possible

We currently have some runtests that skip some platforms (x64, aarch64, or s390x) due to unsupported lowerings. In general, we should support the same set of features everywhere. To the extent that we already have runtests for various corners of CLIF, we should look at each runtest and, where a platform is not included, work out why (missing lowerings or something else) and fix it.

(From discussion on #4139.)