powerpc/math-emu: Adopt new version of _FP_CHOOSENAN

Move to using the same macro definition for _FP_CHOOSENAN as s390,
sh, sparc32/64.  The original author didn't understand this and
matched what sparc64 was doing and they have updated to this definition.

Signed-off-by: Liu Yu <yu.liu@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>