Declare wcstof functions for more _FloatN, _FloatNx types.

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

Tested for x86_64.

	* wcsmbs/wchar.h [__HAVE_FLOAT16 && __USE_GNU] (wcstof16):
	Declare.
	[__HAVE_FLOAT32 && __USE_GNU] (wcstof32): Likewise.
	[__HAVE_FLOAT64 && __USE_GNU] (wcstof64): Likewise.
	[__HAVE_FLOAT32X && __USE_GNU] (wcstof32x): Likewise.
	[__HAVE_FLOAT64X && __USE_GNU] (wcstof64x): Likewise.
	[__HAVE_FLOAT128X && __USE_GNU] (wcstof128x): Likewise.
	[__HAVE_FLOAT16 && __USE_GNU] (wcstof16_l): Likewise.
	[__HAVE_FLOAT32 && __USE_GNU] (wcstof32_l): Likewise.
	[__HAVE_FLOAT64 && __USE_GNU] (wcstof64_l): Likewise.
	[__HAVE_FLOAT32X && __USE_GNU] (wcstof32x_l): Likewise.
	[__HAVE_FLOAT64X && __USE_GNU] (wcstof64x_l): Likewise.
	[__HAVE_FLOAT128X && __USE_GNU] (wcstof128x_l): Likewise.