Define and use libm_alias_ldouble.

Define and use libm_alias_ldouble.

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_ldouble macro and uses
it in the type-generic templates.

Since math-type-macros-ldouble.h already did the appropriate thing for
each symbol (weak_alias or long_double_symbol), this is just a
straightforward rearrangement of code, to make the required logic
available in a place that can also be used outside of the type-generic
templates in future (in particular, to eliminate various wrappers for
functions in ldbl-opt and ldbl-64-128).

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-ldouble.h: New file.
	* sysdeps/ieee754/ldbl-opt/libm-alias-ldouble.h: Likewise.
	* sysdeps/ieee754/ldbl-opt/math-type-macros-ldouble.h: Remove.
	* sysdeps/generic/math-type-macros-ldouble.h: Include
	<libm-alias-ldouble.h>.
	[!declare_mgen_alias] (declare_mgen_alias): Define to use
	libm_alias_ldouble.