Fix the C++ version of issignaling when __NO_LONG_DOUBLE_MATH is defined

When __NO_LONG_DOUBLE_MATH is defined, __issignalingl is not available,
thus issignaling with long double argument should call __issignaling,
instead.

Tested for powerpc64le.

	* math/math.h [defined __cplusplus] (issignaling): In the long
	double case, call __issignalingl only if __NO_LONG_DOUBLE_MATH
	is not defined.  Call __issignaling, otherwise.

(cherry picked from commit 3d7b66f6)