Add C++11 <cmath> overloads to the global namespace

Add C++11 <cmath> overloads to the global namespace

	PR libstdc++/60401
	* include/c_compatibility/math.h (acosh, asinh, atanh, acbrt,
	copysign, erf, erfc, exp2, expm1, fdim, fma, fmax, fmin, hypot, ilogb,
	lgamma, llrint, llround, log1p, log2, logb, lrint, lround, nearbyint,
	nextafter, nexttoward, remainder, remquo, rint, round, scalbln, scalbn,
	tgamma, trunc) [__cplusplus >= 201103L && _GLIBCXX_USE_C99_MATH_TR1]:
	Add using declarations.
	* testsuite/26_numerics/headers/cmath/60401.cc: New.

From-SVN: r232627