Linux: Use __pthread_attr_setsigmask_internal for timer helper thread

timer_create needs to create threads with all signals blocked,
including SIGTIMER (which happens to equal SIGCANCEL).

Fixes commit b3cae39d ("nptl: Start
new threads with all signals blocked [BZ #25098]").

Reviewed-by: Carlos O'Donell <carlos@redhat.com>