Support strtof128 etc. aliases.

This patch adds support for building strtof128, wcstof128, strtof128_l
and wcstof128_l as aliases, 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/strtold.c: Include <bits/floatn.h>
	[__HAVE_FLOAT128 && !__HAVE_DISTINCT_FLOAT128] (strtof128): Define
	and later undefine as macro.  Define as weak alias if
	[!USE_WIDE_CHAR].
	[__HAVE_FLOAT128 && !__HAVE_DISTINCT_FLOAT128] (wcstof128): Define
	and later undefine as macro.  Define as weak alias if
	[USE_WIDE_CHAR].
	* sysdeps/ieee754/ldbl-128/strtold_l.c [__HAVE_FLOAT128 &&
	!__HAVE_DISTINCT_FLOAT128] (strtof128_l): Define and later
	undefine as macro.  Define as weak alias if [!USE_WIDE_CHAR].
	[__HAVE_FLOAT128 && !__HAVE_DISTINCT_FLOAT128] (wcstof128_l):
	Define and later undefine as macro.  Define as weak alias if
	[USE_WIDE_CHAR].
	* sysdeps/ieee754/ldbl-64-128/strtold_l.c: Include
	<bits/floatn.h>.
	[__HAVE_FLOAT128 && !__HAVE_DISTINCT_FLOAT128] (strtof128_l):
	Define and later undefine as macro.  Define as weak alias if
	[!USE_WIDE_CHAR].
	[__HAVE_FLOAT128 && !__HAVE_DISTINCT_FLOAT128] (wcstof128_l):
	Define and later undefine as macro.  Define as weak alias if
	[USE_WIDE_CHAR].