Support strfromf128 alias.

This patch adds support for building strfromf128 as an alias of
strfroml, in the case of __HAVE_FLOAT128 && !__HAVE_DISTINCT_FLOAT128.

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.

	* stdlib/strfroml.c: Include <bits/floatn.h>.
	[__HAVE_FLOAT128 && !__HAVE_DISTINCT_FLOAT128] (strfromf128):
	Define before include of <stdlib.h> and undefine afterwards, then
	define as weak alias.