Add TargetIsa::as_any for downcasting to specific ISA implementations

This is necessary when we would like to check specific ISA flags, e.g.

cranelift/codegen/src/isa/mod.rs

@@ -66,6 +66,7 @@ use crate::settings::SetResult;
 use crate::timing;
 use alloc::borrow::Cow;
 use alloc::boxed::Box;
+use core::any::Any;
 use core::fmt;
 use core::fmt::{Debug, Formatter};
 use target_lexicon::{triple, Architecture, PointerWidth, Triple};
@@ -422,6 +423,10 @@ pub trait TargetIsa: fmt::Display + Send + Sync {
     fn get_mach_backend(&self) -> Option<&dyn MachBackend> {
         None
     }
+
+    /// Return an [Any] reference for downcasting to the ISA-specific implementation of this trait
+    /// with `isa.as_any().downcast_ref::<isa::foo::Isa>()`.
+    fn as_any(&self) -> &dyn Any;
 }
 
 impl Debug for &dyn TargetIsa {