[POWERPC] Move to runtime allocated exception stacks

For the additonal exception levels (critical, debug, machine check) on
40x/book-e we were using "static" allocations of the stack in the
associated head.S.

Move to a runtime allocation to make the code a bit easier to read as
we mimic how we handle IRQ stacks.  Its also a bit easier to setup the
stack with a "dummy" thread_info in C code.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/head_44x.S

@@ -737,15 +737,6 @@ empty_zero_page:
 swapper_pg_dir:
 	.space	PGD_TABLE_SIZE
 
-/* Reserved 4k for the critical exception stack & 4k for the machine
- * check stack per CPU for kernel mode exceptions */
-	.section .bss
-        .align 12
-exception_stack_bottom:
-	.space	BOOKE_EXCEPTION_STACK_SIZE
-	.globl	exception_stack_top
-exception_stack_top:
-
 /*
  * Room for two PTE pointers, usually the kernel and current user pointers
  * to their respective root page table.