integrate 'make check_all_arches' in the Travis pipeline

.travis-ci.sh

@@ -40,6 +40,10 @@ EOF
     make install
     (cd testsuite && make all)
     (cd testsuite && make USE_RUNTIME="d" all)
+    # check_all_arches checks tries to compile all backends in place,
+    # we need to redo (small parts of) world.opt afterwards
+    make check_all_arches
+    make world.opt
     mkdir external-packages
     cd external-packages
     git clone git://github.com/ocaml/ocamlbuild