This allows the `wasmtime` binary provided in our release artifacts to
cross-compile: `wasmtime compile` can build a `.cwasm` for any platform
that Wasmtime supports, not just the host platform. This may be useful
in some deployment scenarios.
We don't turn on `all-arch` by default because it increases build time
and binary size of Wasmtime itself, and other embedders of the
`wasmtime` crate won't necessarily want this; hence, we set it only as
part of the CI build configuration.
Fixes#5655.
