Define and use libm_alias_float128.

Continuing the process of setting up common macros for libm function
aliases, with a view to using them to define _FloatN / _FloatNx
aliases in future, this patch adds a libm_alias_float128 macro and
uses it in the type-generic templates.  (_Float128 functions will end
up with _Float64x aliases on powerpc64le, but not on x86_64/x86/ia64
because _Float64x has long double format there, and the macro will
provide a single place for the conditionals for that choice, as well
as for once ldbl-128 functions always build *f128 and need
conditionals for whether to have *l aliases.)

Tested for x86_64.  Also tested with build-many-glibcs.py that
installed stripped shared libraries are unchanged by the patch.

	* sysdeps/generic/libm-alias-float128.h: New file.
	* sysdeps/generic/math-type-macros-float128.h: Include
	<libm-alias-float128.h>.
	[!declare_mgen_alias] (declare_mgen_alias): Define macro.