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.
