Where can I find a full description of functions available in RVCT libraries ? I only found reference for CARM.
Those functions marked as __ARM_xxxx are internal helper functions that are called by the standard C99 macros. I really doubt that you will find any documentation on those, unless you get the full source code for the libraries.
From the math.h description, the __ARM_fpclassifyf() function returns whether the passed float is a NaN, infinite, normal, subnormal or zero float.
If you really need badly to hack this, just offer some float constants to it, to map what values are returned for each float type.
One word of caution: the internal functions are not guaranteed to be preserved in any way across lib versions. Watch out.