Allow disabling the host-arch feature of cranelift-codegen

[bjorn3]

This is required to compile for a target which doesn't have a cranelift backend. Before this change using any of the cranelift crates that depend on cranelift-codegen would forcefully enable all default features and thus host-arch. With this change only the std and unwind features are still forcefully enabled as cranelift-codegen doesn't compile with either disabled.

Required for rust-lang/rust#81746

[bjorn3]

Without the unwind feature enabled I get:

error[E0433]: failed to resolve: could not find `systemv` in `unwind`
    --> cranelift/codegen/src/machinst/vcode.rs:1181:41
     |
1181 |                     crate::isa::unwind::systemv::caller_sp_to_cfa_offset();
     |                                         ^^^^^^^ could not find `systemv` in `unwind`

[bjorn3]

It would be great if this could get backported too. Without this rust's CI fails when trying to build cg_clif for distribution with rustup.

[alexcrichton]

Thanks! I'm a little surprised that the dependency from wasmtime-cranelift on the cranelift crate, without features = ['default'], still works. I think it's because without any enabled ISAs it still builds the host ISA? Mind adding it there though to be safe?

Otherwise yeah looks like disabling the unwind feature and not being able to compile is indeed a Cranelift bug. If you'd like feel free to open an issue (or a subsequent PR) to fix that. Enabling it here by default is definitely ok.

Also a point release is fine by me, and there's one slated tomorrow for #7362 as well so good timing!

If you wouldn't mind preparing the PR for the backport that'd be appreciated!

[bjorn3]

Mind adding it there though to be safe?

Done

If you'd like feel free to open an issue (or a subsequent PR) to fix that.

Sure

If you wouldn't mind preparing the PR for the backport that'd be appreciated!

Will do.