Only use the -dynamiclib flag when the linker is not ld

This is not a valid flag for ld64. When the ld64 linker is explicitly
provided through `config.toml`, rustc will not successfully compile.

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -315,7 +315,10 @@ impl<'a> GccLinker<'a> {
     fn build_dylib(&mut self, out_filename: &Path) {
         // On mac we need to tell the linker to let this library be rpathed
         if self.sess.target.is_like_osx {
-            self.cmd.arg("-dynamiclib");
+            if !self.is_ld {
+                self.cmd.arg("-dynamiclib");
+            }
+
             self.linker_arg("-dylib");
 
             // Note that the `osx_rpath_install_name` option here is a hack