x86 codegen fix for the release

[Stephen Weeks]

> > The only change is the "if" statement after gcState.canHandle--.
> >
> > 		if (gcState.signalIsPending && 0 == gcState.canHandle)		\
> > 			gcState.limit = gcState.base;				\
> 
> O.k.  Try out
> /net/vinous/home/mfluet/mlton/mlton/src/mlton/codegen-old/x86-codegen/x86-mlton.fun

That works.  Thanks.