boostrap: skip no_std targets in Std doc step

Author: niklaskorz

src/bootstrap/src/core/build_steps/doc.rs

@@ -580,6 +580,10 @@ impl Step for Std {
 
     fn make_run(run: RunConfig<'_>) {
         let crates = compile::std_crates_for_run_make(&run);
+        let target_is_no_std = run.builder.no_std(run.target).unwrap_or(false);
+        if crates.is_empty() && target_is_no_std {
+            return;
+        }
         run.builder.ensure(Std {
             stage: run.builder.top_stage,
             target: run.target,