x86: Move TARGET_SMALL_REGISTER_CLASSES_FOR_MODE_P to i386.cc

Move the TARGET_SMALL_REGISTER_CLASSES_FOR_MODE_P target hook from
i386.h to i386.cc.

	* config/i386/i386.h (TARGET_SMALL_REGISTER_CLASSES_FOR_MODE_P):
	Moved to ...
	* config/i386/i386.cc (TARGET_SMALL_REGISTER_CLASSES_FOR_MODE_P):
	Here.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>