[Patch / RFA] Use builtins for the classification macros

[Paolo Carlini]

Richard Guenther wrote:
> I'll have a look tomorrow.
>   
Thanks a lot Richard. By the way, I decided to add today's improvement
together with fixing libstdc++/25913 itself. Which means, all the
functions in namespace std will be guarded with enable_if and, likely,
tomorrow the testcase will not fail anymore for you. However, preparing
a reproducer should be absolutely trivial: just feed __builtin_isnormal
a user defined type...

Paolo.