Support strtof64x, wcstof64x aliases.

Support strtof64x, wcstof64x aliases.

This patch adds support for defining strtof64x, strtof64x_l, wcstof64
and wcstof64x_l function aliases 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/strtold.c [__HAVE_FLOAT64X_LONG_DOUBLE] (strtof64x):
	Define and later undefine as macro.  Define as weak alias if
	[!USE_WIDE_CHAR].
	[__HAVE_FLOAT64X_LONG_DOUBLE] (wcstof64x): Define and later
	undefine as macro.  Define as weak alias if [USE_WIDE_CHAR].
	* sysdeps/ieee754/float128/strtof128.c: Include <bits/floatn.h>.
	[__HAVE_FLOAT64X && !__HAVE_FLOAT64X_LONG_DOUBLE] (strtof64x):
	Define and later undefine as macro.  Define as weak alias if
	[!USE_WIDE_CHAR].
	[__HAVE_FLOAT64X && !__HAVE_FLOAT64X_LONG_DOUBLE] (wcstof64x):
	Define and later undefine as macro.  Define as weak alias if
	[USE_WIDE_CHAR].
	* sysdeps/ieee754/float128/strtof128_l.c
	[__HAVE_FLOAT64X && !__HAVE_FLOAT64X_LONG_DOUBLE] (strtof64x_l):
	Define and later undefine as macro.  Define as weak alias if
	[!USE_WIDE_CHAR].
	[__HAVE_FLOAT64X && !__HAVE_FLOAT64X_LONG_DOUBLE] (wcstof64x_l):
	Define and later undefine as macro.  Define as weak alias if
	[USE_WIDE_CHAR].
	* sysdeps/ieee754/ldbl-128/strtold_l.c
	[__HAVE_FLOAT64X_LONG_DOUBLE] (strtof64x_l): Define and later
	undefine as macro.  Define as weak alias if [!USE_WIDE_CHAR].
	[__HAVE_FLOAT64X_LONG_DOUBLE] (wcstof64x_l): Define and later
	undefine as macro.  Define as weak alias if [USE_WIDE_CHAR].
	* sysdeps/ieee754/ldbl-64-128/strtold_l.c
	[__HAVE_FLOAT64X_LONG_DOUBLE] (strtof64x_l): Define and later
	undefine as macro.  Define as weak alias if [!USE_WIDE_CHAR].
	[__HAVE_FLOAT64X_LONG_DOUBLE] (wcstof64x_l): Define and later
	undefine as macro.  Define as weak alias if [USE_WIDE_CHAR].
	* sysdeps/ieee754/ldbl-96/strtold_l.c
	[__HAVE_FLOAT64X_LONG_DOUBLE] (strtof64x_l): Define and later
	undefine as macro.  Define as weak alias if [!USE_WIDE_CHAR].
	[__HAVE_FLOAT64X_LONG_DOUBLE] (wcstof64x_l): Define and later
	undefine as macro.  Define as weak alias if [USE_WIDE_CHAR].