Move SNAN_TESTS_PRESERVE_PAYLOAD out of math-tests.h.

Continuing moving macros out of math-tests.h to smaller headers
following typo-proof conventions instead of using #ifndef, this patch
moves SNAN_TESTS_PRESERVE_PAYLOAD out to its own sysdeps header.

Tested with build-many-glibcs.py.

	* sysdeps/generic/math-tests-snan-payload.h: New file.
	* sysdeps/hppa/math-tests-snan-payload.h: Likewise.
	* sysdeps/mips/math-tests-snan-payload.h: Likewise.
	* sysdeps/riscv/math-tests-snan-payload.h: Likewise.
	* sysdeps/generic/math-tests.h: Include
	<math-tests-snan-payload.h>.
	(SNAN_TESTS_PRESERVE_PAYLOAD): Do not define macro here.
	* sysdeps/hppa/math-tests.h: Remove file.
	* sysdeps/mips/math-tests.h [!__mips_nan2008]
	(SNAN_TESTS_PRESERVE_PAYLOAD): Do not define macro here.
	* sysdeps/riscv/math-tests.h (SNAN_TESTS_PRESERVE_PAYLOAD):
	Likewise.