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.