allow running x doc on std for no_std targets

Since we now handle library crates properly, there's no need to panic for `no_std`
targets anymore.

`x doc library` now generates documentation for the `alloc` crate from standard library.

Signed-off-by: onur-ozkan <[email protected]>

Author: onur-ozkan

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

@@ -589,6 +589,7 @@ impl Step for Std {
     }
 
     fn make_run(run: RunConfig<'_>) {
+        let crates = compile::std_crates_for_run_make(&run);
         run.builder.ensure(Std {
             stage: run.builder.top_stage,
             target: run.target,
@@ -597,7 +598,7 @@ impl Step for Std {
             } else {
                 DocumentationFormat::Html
             },
-            crates: run.make_run_crates(Alias::Library),
+            crates,
         });
     }
 
@@ -695,13 +696,6 @@ fn doc_std(
     extra_args: &[&str],
     requested_crates: &[String],
 ) {
-    if builder.no_std(target) == Some(true) {
-        panic!(
-            "building std documentation for no_std target {target} is not supported\n\
-             Set `docs = false` in the config to disable documentation, or pass `--skip library`."
-        );
-    }
-
     let compiler = builder.compiler(stage, builder.config.build);
 
     let target_doc_dir_name = if format == DocumentationFormat::Json { "json-doc" } else { "doc" };