float128: Add _Float128 make bits to libm.

This adds the appropriate common bits for a platform to
enable float128 and expose ABI.

	* math/Makefile:
	(type-float128-suffix): New variable
	(type-float128-routines): Likewise
	(type-float128-yes): Likewise
	(types): Append float128 if supported
	(types-basic): New variable to control the use of templates for
	float, double, and long double, but not for float128 or newer types.
	(type-basic-foreach): Likewise.

	* sysdeps/ieee754/float128/Makeconfig: New file.
	* sysdeps/ieee754/float128/Versions: New file.