The macro __MATH_TG contains the logic to select between long double and
_Float128, when these types are ABI-distinct. This logic relies on
__builtin_types_compatible_p, which is not available in C++ mode.
On the other hand, C++ function overloading provides the means to
distinguish between the floating-point types. The overloading
resolution will match the correct parameter regardless of type
qualifiers, i.e.: const and volatile.
Tested for powerpc64le, s390x, and x86_64.
* math/math.h [defined __cplusplus] (issignaling): Provide a C++
definition for issignaling that does not rely on __MATH_TG,
since __MATH_TG uses __builtin_types_compatible_p, which is only
available in C mode.
(CFLAGS-test-math-issignaling.cc): New variable.
* math/Makefile [CXX] (tests): Add test-math-issignaling.
* math/test-math-issignaling.cc: New test for C++ implementation
of type-generic issignaling.
* sysdeps/powerpc/powerpc64le/Makefile [subdir == math]
(CXXFLAGS-test-math-issignaling.cc): Add -mfloat128 to the build
options of test-math-issignaling on powerpc64le.
