build(crossruby): clarify dump_ast configure arg

Add a short comment explaining why cross builds pass the host-built dump_ast to configure.

Author: ledsun

lib/ruby_wasm/build/product/crossruby.rb

@@ -340,6 +340,8 @@ def configure_args(build_triple, toolchain)
       args << %Q(--with-zlib-dir=#{@zlib.install_root})
       args << %Q(--with-openssl-dir=#{@openssl.install_root}) if @openssl
       args << %Q(--with-baseruby=#{baseruby_path})
+      # Use the host-built dump_ast so cross builds don't try to execute the
+      # target-side wasm dump_ast while generating .rbinc files.
       args << %Q(--with-dump-ast=#{dump_ast_path})
 
       case target