cpu-exec: tighten barrier on TCG_EXIT_REQUESTED

This seems to have worked just fine so far on weakly-ordered
architectures, but I don't see anything that prevents the
reordering from:

    store 1 to exit_request
    store 1 to tcg_exit_req
                                 load tcg_exit_req
                                 store 0 to tcg_exit_req
                                 load exit_request
                                 store 0 to exit_request
    store 1 to exit_request
    store 1 to tcg_exit_req

to this:

    store 1 to exit_request
    store 1 to tcg_exit_req
                                 load tcg_exit_req
                                 load exit_request
    store 1 to exit_request
    store 1 to tcg_exit_req
                                 store 0 to tcg_exit_req
                                 store 0 to exit_request

therefore losing a request.  It's possible that other memory barriers
(e.g. in rcu_read_unlock) are hiding it, but better safe than
sorry.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
