arm64: ptrace: Fix overwriting 1st argument with return value

Since arm64 shares the return value with the area of
the first argument, rewriting the return value before
the system call execution completes destroys the first argument.

Change-Id: I959944879254d8dd3a29489a65d8f274d45338e6
Fujitsu: POSTK_DEBUG_ARCH_DEP_110

kernel/include/syscall.h

@@ -586,6 +586,10 @@ typedef struct uti_attr {
 	uint64_t flags; /* Representing location and behavior hints by bitmap */
 } uti_attr_t;
 
+struct thread;
+long arch_ptrace_syscall_event(struct thread *thread,
+			       ihk_mc_user_context_t *ctx, long setret);
+
 struct uti_ctx {
 	union {
 		char ctx[4096];