Run `cargo miri test` on wasm and riscv target once they're supported

Currently, Miri doesn't support any wasm target (see https://github.com/google/zerocopy/issues/22#issuecomment-1281481417). Once one is supported, we should run `cargo miri test` on a wasm target in CI.

EDIT: This [appears to also be true of riscv64gc-unknown-linux-gnu](https://github.com/google/zerocopy/actions/runs/5883275147/job/15955625276?pr=269), although I haven't found it documented anywhere.

## Old issue text

[This job](https://github.com/google/zerocopy/actions/runs/3185884137/jobs/5195943373) failed for reasons that seem specific to running `cargo miri test` with the target set to `wasm32-wasi`. I'm going to disable the test for the time being.

Thanks to @frazar for discovering https://github.com/rust-lang/miri/issues/1916, which suggests that Miri lacks support for some wasm32 targets. The task for this issue now is to a) determine whether any wasm32 targets are supported, b) switch to using a supported one if so or, c) skip running Miri when targetting wasm32 if none are supported.