Restrict arch as it does not compile for now

Author: balat

opam

@@ -11,6 +11,7 @@ dev-repo: "git+https://github.com/ocsigen/ocsigen-toolkit.git"
 license: "LGPL-2.1-only WITH OCaml-LGPL-linking-exception"
 build: [ make "-j%{jobs}%" ]
 install: [ make "install" ]
+available: arch != "x86_32" & arch != "arm32"
 depends: [
   "ocaml" {>= "4.08.0"}
   "js_of_ocaml" {>= "5.5.0"}