Move SNAN_TESTS_TYPE_CAST out of math-tests.h.

Move SNAN_TESTS_TYPE_CAST out of math-tests.h.

The math-tests.h header has many different macros and groups of
macros, defined using #ifndef in the generic version which is included
by architecture versions with #include_next after possibly defining
non-default versions of some of those macros.

This use of #ifndef is contrary to our normal typo-proof conventions
for macro definitions.  This patch moves one of the macros,
SNAN_TESTS_TYPE_CAST, out to its own sysdeps header, to follow those
typo-proof conventions more closely.

Tested with build-many-glibcs.py.

2018-08-01  Joseph Myers  <joseph@codesourcery.com>

	* sysdeps/generic/math-tests-snan-cast.h: New file.
	* sysdeps/powerpc/math-tests-snan-cast.h: Likewise.
	* sysdeps/generic/math-tests.h: Include <math-tests-snan-cast.h>.
	(SNAN_TESTS_TYPE_CAST): Do not define macro here.
	* sysdeps/powerpc/math-tests.h (SNAN_TESTS_TYPE_CAST): Likewise.