Don't export non-function symbols with emscripten

Emscripten only provides an export mechanism for functions.
Exporting statics does not make sense conceptually in this case,
and will result in emcc undefined function errors.

src/librustc_codegen_utils/symbol_export.rs

@@ -388,6 +388,16 @@ fn symbol_export_level(tcx: TyCtxt, sym_def_id: DefId) -> SymbolExportLevel {
         codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL);
 
     if is_extern && !std_internal {
+        // Emscripten cannot export statics, so reduce their export level here
+        if tcx.sess.target.target.options.is_like_emscripten {
+            if let Some(Node::Item(&hir::Item {
+                node: hir::ItemKind::Static(..),
+                ..
+            })) = tcx.hir.get_if_local(sym_def_id) {
+                return SymbolExportLevel::Rust;
+            }
+        }
+
         SymbolExportLevel::C
     } else {
         SymbolExportLevel::Rust