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/x86/mod.rs

@@ -23,6 +23,7 @@ use crate::result::CodegenResult;
 use crate::timing;
 use alloc::borrow::Cow;
 use alloc::boxed::Box;
+use core::any::Any;
 use core::fmt;
 use target_lexicon::{PointerWidth, Triple};
 
@@ -184,6 +185,10 @@ impl TargetIsa for Isa {
     fn create_systemv_cie(&self) -> Option<gimli::write::CommonInformationEntry> {
         Some(unwind::systemv::create_cie())
     }
+
+    fn as_any(&self) -> &dyn Any {
+        self as &dyn Any
+    }
 }
 
 impl fmt::Display for Isa {