Split helper classification macros from mathcalls.h

Split helper classification macros from mathcalls.h

The classification macros: finite, fpclassify, iseqsig, isinf, isnan,
issignaling, and signbit are defined by ISO C11 and declared in
mathcalls.h for each of the floating-point types: float, double, and
long double.

TS 18661-3 does not mention these macros for float128, however support
for them must be present when _Float128 is present.  This is true,
even when the feature test macro __STDC_WANT_IEC_60559_TYPES_EXT__ is
false.  Other function declarations in mathcalls.h, on the other hand,
depend on __STDC_WANT_IEC_60559_TYPES_EXT__.

This patch splits the helper functions (__finite, __fpclassify,
__iseqsig, __isinf, __isnan, __issignaling, and __signbit) from
mathcalls.h, so that these helper functions can be declared for
_Float128, even when __STDC_WANT_IEC_60559_TYPES_EXT__ is false.

Tested for powerpc64le, s390x, and x86_64.

	* include/bits/mathcalls-helper-functions.h: New file.
	* math/Makefile (headers): Add bits/mathcalls-helper-functions.h.
	* math/bits/mathcalls.h (__finite, __fpclassify, __iseqsig)
	(__isinf, __isnan, __issignaling, __signbit): Move declarations to
	math/bits/mathcalls-helper-functions.h.
	* math/bits/mathcalls-helper-functions.h: New file.
	* math/math.h: Include bits/mathcalls-helper-functions.h for
	float, double, and long double.