csky: Fixup libgcc unwind error

The struct rt_sigframe is also defined in libgcc/config/csky/linux-unwind.h
of gcc. Although there is no use for the first three word space, we must
keep them the same with linux-unwind.h for member position.

The BUG is found in glibc test with the tst-cancel02.
The BUG is from commit:bf2416829362 of linux-5.2-rc1 merge window.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Signed-off-by: Mao Han <han_mao@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>

arch/csky/kernel/signal.c

@@ -39,6 +39,11 @@ static int save_fpu_state(struct sigcontext __user *sc)
 #endif
 
 struct rt_sigframe {
+	/*
+	 * pad[3] is compatible with the same struct defined in
+	 * gcc/libgcc/config/csky/linux-unwind.h
+	 */
+	int pad[3];
 	struct siginfo info;
 	struct ucontext uc;
 };