Fix cpu_unlink_tb race

If a signal hit after the env->exit_request check but before cpu_exec
updated env->current_tb, cpu_unlink_tb called from the signal hander
will not unlink the current TB. This may leave us stuck in a guest loop
if no further unlink is invoked.

Fix this by reordering current_tb update and exit_request check,
additionally enforcing the correct order via a compiler barrier.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
