[plt-scheme] fix for (sin 12345678901234567890123)

> MzScheme v202 for Windows was compiled with Visual Studio .NET (VC7), so
> I'd guess earlier Service Packs for VC6 won't help.

You mean you're not just stuck with the last compiler version from the last
time you shelled out for an MSDN subscription, like I am?  ;)  I mainly
mentioned the service pack to avoid any wild goose chases in the event that
you weren't using the absolute latest version.

I mentioned in my last c.l.s. post that the Microsoft KB entry I referenced
doesn't seem to be exactly what's happening here, although it seems similar.
In particular, using /Og alone (global optimizations) ought to cause the
bug, according to the KB entry, but it doesn't.  However, using the /O2 /Og-
compiler options fixed the problem, as did omitting the /O2 option.  I'd bet
something similar would work with VC7.

Anton