Make the non-threaded stub for pthread_sigmask() a no-op instead
of calling sigprocmask(). This matches the behaviour of thr_sigsetmask()
on Solaris; _pthread_sigmask_stub was added purely for compatibility
with Solaris (for TI-RPC), so it might as well do the same thing.

This fixes the problem where client RPC calls ignored all signals
for the complete duration of the RPC. This behaviour is currently
necessary in the threaded case due to locking issues, but was never
intended to occur in non-threaded programs.

Reviewed by: deischen