Add SNAN macros for more _FloatN, _FloatNx types.

This patch continues the preparation for additional _FloatN / _FloatNx
type support by adding appropriately conditional definitions of SNAN*
macros, corresponding to the SNANF128 definition already present.

Tested for x86_64.

	* math/math.h [__HAVE_FLOAT16 && __GLIBC_USE
	(IEC_60559_TYPES_EXT)] (SNANF16): New macro.
	[__HAVE_FLOAT32 && __GLIBC_USE (IEC_60559_TYPES_EXT)] (SNANF32):
	Likewise.
	[__HAVE_FLOAT64 && __GLIBC_USE (IEC_60559_TYPES_EXT)] (SNANF64):
	Likewise.
	[__HAVE_FLOAT32X && __GLIBC_USE (IEC_60559_TYPES_EXT)] (SNANF32X):
	Likewise.
	[__HAVE_FLOAT64X && __GLIBC_USE (IEC_60559_TYPES_EXT)] (SNANF64X):
	Likewise.
	[__HAVE_FLOAT128X && __GLIBC_USE (IEC_60559_TYPES_EXT)]
	(SNANF128X): Likewise.