nptl: Do not set signal mask on second setjmp return [BZ #28607]

__libc_signal_restore_set was in the wrong place: It also ran
when setjmp returned the second time (after pthread_exit or
pthread_cancel).  This is observable with blocked pending
signals during thread exit.

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

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
(cherry picked from commit e186fc5a)