stage2/wasm: write exports on flush, cleanup

Exports now have a dirty flag and are rewritten on flush if this flag
has been set.

A couple other minor changes have been made based on Andrew's review.

src-self-hosted/Module.zig

@@ -974,7 +974,7 @@ pub fn update(self: *Module) !void {
     }
 
     // This is needed before reading the error flags.
-    try self.bin_file.flush();
+    try self.bin_file.flush(self);
 
     self.link_error_flags = self.bin_file.errorFlags();