Fix warning on new-stable (#3131)

One of the fields of `TargetIsa` isn't used in the
cranelift-codegen-meta crate, but instead of refactoring to try to
remove it this just adds `#[allow(dead_code)]` for now in the assumption
that when the old backends go away this will probably go away as well.

cranelift/codegen/meta/src/cdsl/isa.rs

@@ -10,6 +10,7 @@ use crate::cdsl::xform::{TransformGroupIndex, TransformGroups};
 
 pub(crate) struct TargetIsa {
     pub name: &'static str,
+    #[allow(dead_code)]
     pub instructions: InstructionGroup,
     pub settings: SettingGroup,
     pub regs: IsaRegs,