[lldb] [MainLoop] Initialize empty sigset_t correctly

Fix MainLoop::RunImpl::get_sigmask() to correctly return empty sigset_t
when SIGNAL_POLLING_UNSUPPORTED is true.  On NetBSD (and probably
on some other platforms), integers are not implicitly convertible to
sigset_t, so 'return 0' is erraneous.  Instead, sigset_t should be reset
through sigemptyset().

While at it, move common parts out of the #ifdef.

Differential Revision: https://reviews.llvm.org/D57959

llvm-svn: 353675