Add SNAN, SNANF, SNANL macros.

TS 18661-1 defines SNAN macros for signaling NaN values, suitable for
use in static initializers.  This patch adds them to glibc's <math.h>
(provided you are building with GCC 3.3 or later; no attempt is made
to provide any kind of nonconforming fallback for older compilers
without the __builtin_nans functions).

Tested for x86_64 and x86.

	* math/math.h
	[__GLIBC_USE (IEC_60559_BFP_EXT) && __GNUC_PREREQ (3, 3)] (SNANF):
	New macro.
	[__GLIBC_USE (IEC_60559_BFP_EXT) && __GNUC_PREREQ (3, 3)] (SNAN):
	Likewise.
	[__GLIBC_USE (IEC_60559_BFP_EXT) && __GNUC_PREREQ (3, 3)] (SNANL):
	Likewise.
	* manual/arith.texi (Infinity and NaN): Document SNANF, SNAN and
	SNANL.
	* math/test-double.h (snan_value_MACRO): New macro.
	* math/test-float.h (snan_value_MACRO): Likewise.
	* math/test-ldouble.h (snan_value_MACRO): Likewise.
	* math/libm-test.inc (issignaling_test_data): Add tests of
	snan_value_MACRO.