csky: Fixup abiv2 syscall_trace break a4 & a5

Current implementation could destory a4 & a5 when strace, so we need to get them
from pt_regs by SAVE_ALL.
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>

arch/csky/abiv2/inc/abi/entry.h

@@ -13,6 +13,8 @@
 #define LSAVE_A1	28
 #define LSAVE_A2	32
 #define LSAVE_A3	36
+#define LSAVE_A4	40
+#define LSAVE_A5	44
 
 #define KSPTOUSP
 #define USPTOKSP

arch/csky/kernel/entry.S

@@ -173,8 +173,10 @@ csky_syscall_trace:
 	ldw	a3, (sp, LSAVE_A3)
 #if defined(__CSKYABIV2__)
 	subi	sp, 8
-	stw	r5, (sp, 0x4)
-	stw	r4, (sp, 0x0)
+	ldw	r9, (sp, LSAVE_A4)
+	stw	r9, (sp, 0x0)
+	ldw	r9, (sp, LSAVE_A5)
+	stw	r9, (sp, 0x4)
 #else
 	ldw	r6, (sp, LSAVE_A4)
 	ldw	r7, (sp, LSAVE_A5)