eco32: change syscall ABI to hold syscall number in $2

This would have been a nasty change if we actually had any
eco32 Linux users, luckily we don't (yet).

There are a couple of reasons why to do this, the two main are:
1) This frees up $25 for other usages
2) We can get rid of that ugly work-around with the ori

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