util: Predicate the fpclassify fallback on !defined(__cplusplus)

util: Predicate the fpclassify fallback on !defined(__cplusplus)

The problem is that the fallbacks we have at the moment don't work in C++.
While we could theoretically fix the fallbacks it would also raise the
issue of correctly detecting the fpclassify function.  So, for now, we'll
just disable it until we actually have a C++ user.

Reported-by: Tom Stellard <thomas.stellard@amd.com>
Tested-by: Tom Stellard <thomas.stellard@amd.com>
Tested-by: EdB <edb+mesa@sigluy.net>