Support defining strtof64, strtof32x, wcstof64, wcstof32x aliases.

Support defining strtof64, strtof32x, wcstof64, wcstof32x aliases.

This patch adds support for defining strtof64, strtof32x, wcstof64,
wcstof32x and the corresponding _l functions as aliases of the
corresponding double functions when _Float64 and _Float32x support is
enabled.

Tested for x86_64; also tested with build-many-glibcs.py in
conjunction with other _Float64 / _Float32x changes.

	* stdlib/strtod.c: Include <bits/floatn.h>.
	(BUILD_DOUBLE): New macro.
	[BUILD_DOUBLE && __HAVE_FLOAT64 && !__HAVE_DISTINCT_FLOAT64]
	(strtof64): Define and later undefine as macro.  Define as weak
	alias if [!USE_WIDE_CHAR].
	[BUILD_DOUBLE && __HAVE_FLOAT64 && !__HAVE_DISTINCT_FLOAT64]
	(wcstof64): Define and later undefine as macro.  Define as weak
	alias if [USE_WIDE_CHAR].
	[BUILD_DOUBLE && __HAVE_FLOAT32X && !__HAVE_DISTINCT_FLOAT32X]
	(strtof32x): Define and later undefine as macro.  Define as weak
	alias if [!USE_WIDE_CHAR].
	[BUILD_DOUBLE && __HAVE_FLOAT32X && !__HAVE_DISTINCT_FLOAT32X]
	(wcstof32x): Define and later undefine as macro.  Define as weak
	alias if [USE_WIDE_CHAR].
	* stdlib/strtod_l.c: Include <bits/floatn.h>.
	(BUILD_DOUBLE): New macro.
	[BUILD_DOUBLE && __HAVE_FLOAT64 && !__HAVE_DISTINCT_FLOAT64]
	(strtof64_l): Define and later undefine as macro.  Define as weak
	alias if [!USE_WIDE_CHAR].
	[BUILD_DOUBLE && __HAVE_FLOAT64 && !__HAVE_DISTINCT_FLOAT64]
	(wcstof64_l): Define and later undefine as macro.  Define as weak
	alias if [USE_WIDE_CHAR].
	[BUILD_DOUBLE && __HAVE_FLOAT32X && !__HAVE_DISTINCT_FLOAT32X]
	(strtof32x_l): Define and later undefine as macro.  Define as weak
	alias if [!USE_WIDE_CHAR].
	[BUILD_DOUBLE && __HAVE_FLOAT32X && !__HAVE_DISTINCT_FLOAT32X]
	(wcstof32x_l): Define and later undefine as macro.  Define as weak
	alias if [USE_WIDE_CHAR].