Add emscripten/wasm to dox builds

src/lib.rs

@@ -72,6 +72,12 @@
 #![cfg_attr(target_os = "dragonfly", doc(
     html_root_url = "https://doc.rust-lang.org/libc/x86_64-unknown-dragonfly"
 ))]
+#![cfg_attr(all(target_os = "emscripten", target_arch = "asmjs"), doc(
+    html_root_url = "https://doc.rust-lang.org/libc/asmjs-unknown-emscripten"
+))]
+#![cfg_attr(all(target_os = "emscripten", target_arch = "wasm32"), doc(
+    html_root_url = "https://doc.rust-lang.org/libc/wasm32-unknown-emscripten"
+))]
 
 // Attributes needed when building as part of the standard library
 #![cfg_attr(stdbuild, feature(no_std, core, core_slice_ext, staged_api, custom_attribute, cfg_target_vendor))]