Support strfromf64x alias.

Support strfromf64x alias.

This patch adds support for defining strfromf64x as a function alias
(of strfroml or strfromf128, as appropriate) when _Float64x is
supported.

Tested for x86_64, including in conjunction with _Float64x support
patches, and also tested build for other configurations (in
conjunction with _Float64x support patches) with build-many-glibcs.py
to cover the various different files needing updating to define these
aliases.

	* stdlib/strfroml.c: Always include <stdlib.h>.
	[__HAVE_FLOAT64X_LONG_DOUBLE] (strfromf64x): Define and later
	undefine as macro and define as weak alias.
	* sysdeps/ieee754/float128/strfromf128.c: Include <bits/floatn.h>.
	[__HAVE_FLOAT64X && !__HAVE_FLOAT64X_LONG_DOUBLE]: Include
	<stdlib.h>.
	[__HAVE_FLOAT64X && !__HAVE_FLOAT64X_LONG_DOUBLE] (strfromf64x):
	Define and later undefine as macro and define as weak alias.