m68k: build fixes and sparse annotations

Patch series against the mainline; most of it directly applies to m68k CVS, except for #18 (dmasound_paula lvalues abuse) that is pulled from CVS and #4 (misc/mac.c switch away from adb_request()) - that one does apply to CVS but assumes that ADBREQ_RAW patch (present in CVS, but absent in mainline) is reverted.

All patches affect only m68k or (in one patch) ppc/amiga; the latter is a trivial renaming of a macro from amigahw.h. IOW, it should be mergable at any point without any impact on other architectures. And in the current mainline m68k still doesn't build...

Patches follow. Please, review.