csky: stacktrace supported.

The gcc option "-mbacktrace" will push fp(r8),lr into stack and we could
unwind the stack with:
	fp = *fp
	lr = (unsigned int *)fp[1]

Signed-off-by: Guo Ren <ren_guo@c-sky.com>

arch/csky/kernel/stacktrace.c

@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2018 Hangzhou C-SKY Microsystems co.,ltd. */
+
+#include <linux/sched/debug.h>
+#include <linux/sched/task_stack.h>
+#include <linux/stacktrace.h>
+#include <linux/ftrace.h>
+
+void save_stack_trace(struct stack_trace *trace)
+{
+	save_stack_trace_tsk(current, trace);
+}
+EXPORT_SYMBOL_GPL(save_stack_trace);
+
+void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
+{
+	unsigned long *fp, *stack_start, *stack_end;
+	unsigned long addr;
+	int skip = trace->skip;
+	int savesched;
+	int graph_idx = 0;
+
+	if (tsk == current) {
+		asm volatile("mov %0, r8\n":"=r"(fp));
+		savesched = 1;
+	} else {
+		fp = (unsigned long *)thread_saved_fp(tsk);
+		savesched = 0;
+	}
+
+	addr = (unsigned long) fp & THREAD_MASK;
+	stack_start = (unsigned long *) addr;
+	stack_end = (unsigned long *) (addr + THREAD_SIZE);
+
+	while (fp > stack_start && fp < stack_end) {
+		unsigned long lpp, fpp;
+
+		fpp = fp[0];
+		lpp = fp[1];
+		if (!__kernel_text_address(lpp))
+			break;
+		else
+			lpp = ftrace_graph_ret_addr(tsk, &graph_idx, lpp, NULL);
+
+		if (savesched || !in_sched_functions(lpp)) {
+			if (skip) {
+				skip--;
+			} else {
+				trace->entries[trace->nr_entries++] = lpp;
+				if (trace->nr_entries >= trace->max_entries)
+					break;
+			}
+		}
+		fp = (unsigned long *)fpp;
+	}
+}
+EXPORT_SYMBOL_GPL(save_stack_trace_tsk);