kern/94583: [libm] std::isnan()/std::isinf() are not identical to isnan() and isinf()

[David Schultz]

libstdc++ was upgraded from 3.4.6 to 4.2.1 between FreeBSD 6.X and
7.0, and while I don't have a 7.X machine available to test at the
moment, it seems likely that some of the changes between the two
versions fixes the problem.

If anyone else running 7.X can try to the test case attached to
this PR, that would be great.