Remove -sASSERTIONS=1 from wasm32_unknown_emscripten default link args

This is a debug setting. We should only make debug builds if user requests
a debug build. Currently this is inserted in release builds.

Furthermore, it would be better to insert these settings in --pre-link-args
because then it would be possible to override them if appropriate. Because
these are inserted at the end, it is necessary to patch emscripten to remove
them.

compiler/rustc_target/src/spec/wasm32_unknown_emscripten.rs

@@ -20,8 +20,6 @@ pub fn target() -> Target {
             "-s".into(),
             "ERROR_ON_UNDEFINED_SYMBOLS=1".into(),
             "-s".into(),
-            "ASSERTIONS=1".into(),
-            "-s".into(),
             "ABORTING_MALLOC=0".into(),
             "-Wl,--fatal-warnings".into(),
         ],