Add emscripten to the "wasm" family of targets

Author: alexcrichton

compiler/rustc_target/src/spec/wasm32_unknown_emscripten.rs

@@ -37,7 +37,7 @@ pub fn target() -> Target {
         is_like_emscripten: true,
         panic_strategy: PanicStrategy::Unwind,
         post_link_args,
-        families: vec!["unix".to_string()],
+        families: vec!["unix".to_string(), "wasm".to_string()],
         ..options
     };
     Target {