m68k: Kill several external declarations in source files

- Replace external declarations by proper includes where availiable.
    The accesses to some symbols had to be modified, as before they were
    declared using e.g. "extern int _end", while asm-generic/sections.h uses
    e.g. "extern char _end[]"
  - Remove unused or superfluous external declarations

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/sun3/mmu_emu.c

@@ -27,7 +27,6 @@
 #include <asm/mmu_context.h>
 #include <asm/dvma.h>
 
-extern void prom_reboot (char *) __attribute__ ((__noreturn__));
 
 #undef DEBUG_MMU_EMU
 #define DEBUG_PROM_MAPS