ia64: Avoid ".save rp, r0" since the kernel unwinder doesn't support it yet.

Once we switch to a libunwind-based kernel unwinder, this code can be
re-enabled again.

arch/ia64/kernel/head.S

@@ -818,7 +818,8 @@ END(ia64_delay_loop)
 
 GLOBAL_ENTRY(start_kernel_thread)
 	.prologue
-	.save rp, r0				// this is the end of the call-chain
+	.save rp, r4				// this is the end of the call-chain
+	mov r4=r0
 	.body
 	alloc r2 = ar.pfs, 0, 0, 2, 0
 	mov out0 = r9