sh: switch to saved_sigmask-based sigsuspend()/rt_sigsuspend()

Complete the move of sh64 to it, trim the crap from prototypes,
tidy up a bit.  Infrastructure in do_signal() had already been
there, in signal_64 as well as in signal_32 (where it was already
used).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>