Real.maxFinite bug

[Matthew Fluet]

> > Shouldn't Real.isFinite be handled in native code?  Since you have to store
> > the floating point number in memory any way the cost probably isn't horrible,
> > but still it seems a bit strange.
> 
> Primitive.Real.isFinite could be defined as an _prim instead of an _ffi.  I'll
> leave it to Matthew to decide if he can do any better in the native codegen than
> calling the Real_isFinite.

I'll think about it.  We do pay a little more than just banging the float
to memory, because to make a C call, we need to clear the entire floating
point stack, plus other setup.