libstdc++: Fix std::strong_order to handle NaN on VAX

libstdc++: Fix std::strong_order to handle NaN on VAX

I mistakenly believed that VAX floats do not support NaN, but with GCC
__builtin_isnan(__builtin_nan("")) is true. That means my previous
change to <compare> is wrong, because it fails to handle NaN.

When std::numeric_limits<floating-point-type>::is_iec559 is false, as on
VAX, the standard only requires an ordering that is consistent with the
ordering observed by comparison operators. With this change the ordering
is -NaN < numbers < +NaN, and there is no support for different NaN bit
patterns (as I'm not even sure if GCC supports any for VAX).

libstdc++-v3/ChangeLog:

	* libsupc++/compare (_Strong_order::_S_fp_cmp) [__vax__]:
	Handle NaN.