Auto merge of #43348 - kennytm:fix-24658-doc-every-platform, r=alexcrichton

Expose all OS-specific modules in libstd doc.

1. Uses the special `--cfg dox` configuration passed by rustbuild when running `rustdoc`. Changes the `#[cfg(platform)]` into `#[cfg(any(dox, platform))]` so that platform-specific API are visible to rustdoc.

2. Since platform-specific implementations often won't compile correctly on other platforms, `rustdoc` is changed to apply `everybody_loops` to the functions during documentation and doc-test harness.

3. Since platform-specific code are documented on all platforms now, it could confuse users who found a useful API but is non-portable. Also, their examples will be doc-tested, so must be excluded when not testing on the native platform. An undocumented attribute `#[doc(cfg(...))]` is introduced to serve the above purposed.

Fixes #24658 (Does _not_ fully implement #1998).

src/librustdoc/html/render.rs

@@ -1962,6 +1962,14 @@ fn short_stability(item: &clean::Item, cx: &Context, show_reason: bool) -> Vec<S
         stability.push(format!("<div class='stab deprecated'>{}</div>", text))
     }
 
+    if let Some(ref cfg) = item.attrs.cfg {
+        stability.push(format!("<div class='stab portability'>{}</div>", if show_reason {
+            cfg.render_long_html()
+        } else {
+            cfg.render_short_html()
+        }));
+    }
+
     stability
 }