Declare strtof, strfromf functions for more _FloatN, _FloatNx types.

Continuing the preparation for additional _FloatN / _FloatNx type
support, this patch arranges for <stdlib.h> to declare strtof and
strfromf functions for all such types, similarly to the declarations
already present for _Float128.

Tested for x86_64.

	* stdlib/stdlib.h
	[__HAVE_FLOAT16 && __GLIBC_USE (IEC_60559_TYPES_EXT)] (strtof16):
	Declare.
	[__HAVE_FLOAT32 && __GLIBC_USE (IEC_60559_TYPES_EXT)] (strtof32):
	Likewise.
	[__HAVE_FLOAT64 && __GLIBC_USE (IEC_60559_TYPES_EXT)] (strtof64):
	Likewise.
	[__HAVE_FLOAT32X && __GLIBC_USE (IEC_60559_TYPES_EXT)]
	(strtof32x): Likewise.
	[__HAVE_FLOAT64X && __GLIBC_USE (IEC_60559_TYPES_EXT)]
	(strtof64x): Likewise.
	[__HAVE_FLOAT128X && __GLIBC_USE (IEC_60559_TYPES_EXT)]
	(strtof128x): Likewise.
	[__HAVE_FLOAT16 && __GLIBC_USE (IEC_60559_TYPES_EXT)]
	(strfromf16): Likewise.
	[__HAVE_FLOAT32 && __GLIBC_USE (IEC_60559_TYPES_EXT)]
	(strfromf32): Likewise.
	[__HAVE_FLOAT64 && __GLIBC_USE (IEC_60559_TYPES_EXT)]
	(strfromf64): Likewise.
	[__HAVE_FLOAT32X && __GLIBC_USE (IEC_60559_TYPES_EXT)]
	(strfromf32x): Likewise.
	[__HAVE_FLOAT64X && __GLIBC_USE (IEC_60559_TYPES_EXT)]
	(strfromf64x): Likewise.
	[__HAVE_FLOAT128X && __GLIBC_USE (IEC_60559_TYPES_EXT)]
	(strfromf128x): Likewise.
	[__USE_GNU && __HAVE_FLOAT16] (strtof16_l): Likewise.
	[__USE_GNU && __HAVE_FLOAT32] (strtof32_l): Likewise.
	[__USE_GNU && __HAVE_FLOAT64] (strtof64_l): Likewise.
	[__USE_GNU && __HAVE_FLOAT32X] (strtof32x_l): Likewise.
	[__USE_GNU && __HAVE_FLOAT64X] (strtof64x_l): Likewise.
	[__USE_GNU && __HAVE_FLOAT128X] (strtof128x_l): Likewise.