Support _Float128 in math-tests.h.

Support _Float128 in math-tests.h.

This patch makes math-tests.h, as used to describe support of given
floating-point types for sNaNs, rounding modes and exceptions, handle
distinguishing _Float128 from long double.  This is needed for x86_64,
where if building with GCC 6 or earlier there is no __builtin_nansq,
so no way to get a signaling NaN of _Float128 type, so associated
tests cannot be run (although glibc itself works fine, as there is
never any need to create such an sNaN with a built-in function inside
glibc).

Tested for x86_64 (in conjunction with float128 patches).

	* sysdeps/generic/math-tests.h: Include <bits/floatn.h>.
	(MATH_TESTS_TG): New macro.
	(SNAN_TESTS_float128): Likewise.
	(ROUNDING_TESTS_float128): Likewise.
	(EXCEPTION_TESTS_float128): Likewise.
	(SNAN_TESTS): Define using MATH_TESTS_TG.
	(ROUNDING_TESTS): Likewise.
	(EXCEPTION_TESTS): Likewise.