eco32: entry: add convinience macro for setting PSW

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
eco32: entry: documentation and whitespace fix
eco32: entry: fall through to ret_from_syscall from syscll_handler

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
eco32: check thread_info flags for pending work after exceptions

This polls the current_thread_info()->flags and address any
pending work until all of it is handled.

There are several TODOs here:
- Signal handling is still stubbed-out.
- The flags should also be polled after syscalls have been
  handled, they however follow an other path than the rest of
  the exceptions, so it's not included here.

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>