[perl #122788] broken nan on win32

Apparently VS 2003 smartly optimizes (HUGE_VAL-HUGE_VAL) into zero,
so using that for NaN was quite a disaster.  Let instead Win32 fall
thru to the __PL_nan_u option.  (HUGE_VAL for Inf is fine.)
Do not test inf/nan if utterly broken.

(The test for the brokenness was ... broken.)
(I think FC pointed this out some weeks ago.)