[dev.cc] cmd/gc: tweak default fatal in ordersafeexpr for c2go

[dev.cc] cmd/gc: tweak default fatal in ordersafeexpr for c2go

c2go was putting a fallthrough after the fatal call.
Changed c2go to know that fatal doesn't return,
but then there is a missing return at the end of
the translated Go function.
Move code around a little to make C and Go agree.

Change-Id: Icef3d55ccdde0709c02dd0c2b78826f6da33a146
Reviewed-on: https://go-review.googlesource.com/5170
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>