Rollup merge of #131479 - madsmtm:avoid-redundant-dylib, r=jieyouxu

Apple: Avoid redundant `-Wl,-dylib` flag when linking

Seems to have been introduced all the way back in e338a41, but should be redundant, `-dynamiclib` should already make `cc` set `-dylib` when linking.

Spotted this while trying to get `-Clinker-flavor=gcc` and `-Clinker-flavor=ld` closer together, not that important to fix.

`@rustbot` label O-apple

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -404,12 +404,14 @@ impl<'a> GccLinker<'a> {
     fn build_dylib(&mut self, crate_type: CrateType, out_filename: &Path) {
         // On mac we need to tell the linker to let this library be rpathed
         if self.sess.target.is_like_osx {
-            if !self.is_ld {
+            if self.is_cc() {
+                // `-dynamiclib` makes `cc` pass `-dylib` to the linker.
                 self.cc_arg("-dynamiclib");
+            } else {
+                self.link_arg("-dylib");
+                // Clang also sets `-dynamic`, but that's implied by `-dylib`, so unnecessary.
             }
 
-            self.link_arg("-dylib");
-
             // Note that the `osx_rpath_install_name` option here is a hack
             // purely to support bootstrap right now, we should get a more
             // principled solution at some point to force the compiler to pass