[PATCH v3] Provide a C++ version of issignaling that does not use __MATH_TG

[Joseph Myers]

This patch breaks building tests for long double = double configurations.  
__issignalingl does not exist in that case; the long double version of 
issignaling has to call __issignaling if __NO_LONG_DOUBLE_MATH is defined.

-- 
Joseph S. Myers
joseph@codesourcery.com