* defs.h: Move definitions of TARGET_*_BIT after include of tm.h.

	* defs.h: Move definitions of TARGET_*_BIT after include of tm.h.
	The old way (using #undef in tm.h) was ugly and asking for
	trouble, because it makes it possible for some file to use the
	wrong definition.  Move definition of HOST_CHAR_BIT after definition
	of TARGET_CHAR_BIT.
	* config/alpha/tm-alpha.h, config/h8300/tm-h8300.h,
	config/h8500/tm-h8500.h, config/z8k/tm-z8k.h: Don't undef TARGET_*_BIT
	before defining them.