In 1e2b3780 I changed the option
ordering for C compilations.  A side effect was that -optc options
came before the automatic -O we were adding, which made it so that the
-debug RTS was getting optimised when it shouldn't have been.

Perhaps we shouldn't have automatic -O options added to C
compilations.  But that might cause problems for build systems that
are relying on the current behaviour, so I've made a minor change
instead: now C optimisation level == Haskell optimisation level.