Restore building of +sb-unicode with -sb-unicode.

Some host definitions slipped in during cross-compilation.
char-code-limit => sb!xc:char-code-limit.
**character-primary-compositions** was initialized at xc-time, move it to cold init.

Thanks to Christophe Rhodes for the latter.
Remove a level of indirection in *linkage-info*.

Don't store the address in a linkage-info structure, only the linkage-info-address
slot was ever used. Store the address directly.