powerpc/64: add stack protector support

On PPC64, as register r13 points to the paca_struct at all time,
this patch adds a copy of the canary there, which is copied at
task_switch.
That new canary is then used by using the following GCC options:
-mstack-protector-guard=tls
-mstack-protector-guard-reg=r13
-mstack-protector-guard-offset=offsetof(struct paca_struct, canary))

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/asm-offsets.c

@@ -81,6 +81,9 @@ int main(void)
 	OFFSET(MM, task_struct, mm);
 #ifdef CONFIG_STACKPROTECTOR
 	OFFSET(TASK_CANARY, task_struct, stack_canary);
+#ifdef CONFIG_PPC64
+	OFFSET(PACA_CANARY, paca_struct, canary);
+#endif
 #endif
 	OFFSET(MMCONTEXTID, mm_struct, context.id);
 #ifdef CONFIG_PPC64