Handle signature() for more libcalls (#6174)

* Handle signature() for more libcalls

This is necessary to be able to call them in the interpreter. All the
remaining libcalls which signature() doesn't handle are never used in
clif ir. Only in code compiled by a backend.

* Fix libcall declarations in cranelift-frontend

* Add function signatures

* Use correct pointer type instead of I64

cranelift/fuzzgen/src/function_generator.rs

@@ -1863,7 +1863,11 @@ where
             .libcalls
             .iter()
             .map(|libcall| {
-                let signature = libcall.signature(lib_callconv);
+                let pointer_type = Type::int_with_byte_size(
+                    self.target_triple.pointer_width().unwrap().bytes().into(),
+                )
+                .unwrap();
+                let signature = libcall.signature(lib_callconv, pointer_type);
                 let name = ExternalName::LibCall(*libcall);
                 (name, signature)
             })