nptl: SIGCANCEL, SIGTIMER, SIGSETXID are always defined

All nptl targets have these signal definitions nowadays.  This
changes also replaces the nptl-generic version of pthread_sigmask
with the Linux version.

Tested on x86_64-linux-gnu and i686-linux-gnu.  Built with
build-many-glibcs.py.

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