Use libm_alias_ldouble_other in ldbl-64-128/s_nextafterl.c.

This patch makes ldbl-64-128/s_nextafterl.c restore the default
weak_alias definition and use libm_alias_ldouble_other (having
undefined and redefined weak_alias for the include of
ldbl-128/s_nextafterl.c, so the libm_alias_ldouble use in the latter
file is ineffective).

Tested with build-many-glibcs.py that installed stripped shared
libraries are unchanged by this patch.  Also tested together with
changes to enable float128 aliases.

	* sysdeps/ieee754/ldbl-64-128/s_nextafterl.c (weak_alias):
	Undefine and restore default definition.  Use
	libm_alias_ldouble_other.