Add option to pass a custom codegen backend from a driver

compiler/rustc_interface/src/interface.rs

@@ -154,6 +154,10 @@ pub struct Config {
     pub override_queries:
         Option<fn(&Session, &mut ty::query::Providers, &mut ty::query::Providers)>,
 
+    /// This is a callback from the driver that is called to create a codegen backend.
+    pub make_codegen_backend:
+        Option<Box<dyn FnOnce(&config::Options) -> Box<dyn CodegenBackend> + Send>>,
+
     /// Registry of diagnostics codes.
     pub registry: Registry,
 }
@@ -167,6 +171,7 @@ pub fn create_compiler_and_run<R>(config: Config, f: impl FnOnce(&Compiler) -> R
         config.file_loader,
         config.input_path.clone(),
         config.lint_caps,
+        config.make_codegen_backend,
         registry.clone(),
     );