Blackfin: sanitize manual control of IPEND[4]

Cleanup is performed in two ways:

- remove extraneous updates of IPEND[4] w/ CONFIG_IPIPE,
  and document remaining use.

- substitute pop-reg-from-stack instructions with plain SP fixups in
  all save-RETI-then-discard patterns.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>