0.9.18.28: allow read-only space to be placed in higher addresses on PPC

 * Rewrite jumps encoding addresses to read-only space with a spare
   register so that the address range is not limited.
 * This means that fix-rospace magic can go away. (But the real reason for
   this is the forthcoming adjustable dynamic-space-size and
   relocation, which in conjunction with the rospace magic make me
   uncomfortable).