eco32: use generic timex

eco32 doesn't have cycle timer (at least none that can be read),
so remove the bogus support currently in there.
We can't support read_current_timer neither, so remove that as well.

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
eco32: entry: check for pending work in ret_from_syscall

Code duplicated from the exception return,
perhaps this could be made into a macro.
Also, ensure interrupts are turned off up until the rfx.

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
eco32: implement sys_rt_sigreturn

This makes the signal handling pretty complete, there's however
room for improvements still.

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