csky: Add support for restartable sequence

Copied and adapted from vincent's patch, but modified for csky.

ref:
https://lore.kernel.org/linux-riscv/1572919114-3886-3-git-send-email-vincent.chen@sifive.com/raw

Add calls to rseq_signal_deliver(), rseq_handle_notify_resume() and
rseq_syscall() to introduce RSEQ support.

1. Call the rseq_handle_notify_resume() function on return to userspace
   if TIF_NOTIFY_RESUME thread flag is set.

2. Call the rseq_signal_deliver() function to fixup on the pre-signal
   frame when a signal is delivered on top of a restartable sequence
   critical section.

3. Check that system calls are not invoked from within rseq critical
   sections by invoking rseq_signal() from ret_from_syscall(). With
   CONFIG_DEBUG_RSEQ, such behavior results in termination of the
   process with SIGSEGV.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>

arch/csky/kernel/entry.S

@@ -128,7 +128,10 @@ tlbop_end 1
 ENTRY(csky_systemcall)
 	SAVE_ALL TRAP0_SIZE
 	zero_fp
-
+#ifdef CONFIG_RSEQ_DEBUG
+	mov	a0, sp
+	jbsr	rseq_syscall
+#endif
 	psrset  ee, ie
 
 	lrw     r11, __NR_syscalls