Add a default implementation for CodegenBackend::link

As a side effect this should add raw-dylib support to cg_gcc as the
default ArchiveBuilderBuilder that is used implements
create_dll_import_lib. I haven't tested if the raw-dylib support
actually works however.

compiler/rustc_codegen_ssa/src/back/archive.rs

@@ -304,6 +304,14 @@ pub trait ArchiveBuilder {
     fn build(self: Box<Self>, output: &Path) -> bool;
 }
 
+pub struct ArArchiveBuilderBuilder;
+
+impl ArchiveBuilderBuilder for ArArchiveBuilderBuilder {
+    fn new_archive_builder<'a>(&self, sess: &'a Session) -> Box<dyn ArchiveBuilder + 'a> {
+        Box::new(ArArchiveBuilder::new(sess, &DEFAULT_OBJECT_READER))
+    }
+}
+
 #[must_use = "must call build() to finish building the archive"]
 pub struct ArArchiveBuilder<'a> {
     sess: &'a Session,

compiler/rustc_codegen_ssa/src/traits/backend.rs

@@ -16,6 +16,8 @@ use rustc_span::symbol::Symbol;
 
 use super::CodegenObject;
 use super::write::WriteBackendMethods;
+use crate::back::archive::ArArchiveBuilderBuilder;
+use crate::back::link::link_binary;
 use crate::back::write::TargetMachineFactoryFn;
 use crate::{CodegenResults, ModuleCodegen};
 
@@ -87,7 +89,9 @@ pub trait CodegenBackend {
         sess: &Session,
         codegen_results: CodegenResults,
         outputs: &OutputFilenames,
-    ) -> Result<(), ErrorGuaranteed>;
+    ) -> Result<(), ErrorGuaranteed> {
+        link_binary(sess, &ArArchiveBuilderBuilder, &codegen_results, outputs)
+    }
 
     /// Returns `true` if this backend can be safely called from multiple threads.
     ///