powerpc: Move some extern declarations from C code into headers

This also make klimit have the same type on 32-bit as on 64-bit,
namely unsigned long, and defines and initializes it in one place.

Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/mm/init_64.c

@@ -80,8 +80,6 @@
 #warning TASK_SIZE is smaller than it needs to be.
 #endif
 
-unsigned long klimit = (unsigned long)_end;
-
 /* max amount of RAM to use */
 unsigned long __max_memory;