ARC: stop using pt_regs->orig_r8

Historically, pt_regs have had orig_r8, an overloaded container for
  (1) backup copy of r8 (syscall number Trap Exceptions)
  (2) additional system state: (syscall/Exception/Interrupt)

There is no point in keeping (1) since syscall number is never clobbered
in-place, in pt_regs, unlike r0 which duals as first syscall arg as well
as syscall return value and in case of syscall restart, the orig arg0
needs restoring (from orig_r0)  after having been updated in-place with
syscall ret value.

This further paves way to convert (2) to contain ECR itself (rather than
current madeup values)
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/entry.h

@@ -422,18 +422,7 @@
  * Save scratch regs for sys calls
  *-------------------------------------------------------------*/
 .macro SAVE_ALL_TRAP
-	/*
-	 * Setup pt_regs->orig_r8.
-	 * Encode syscall number (r8) in upper short word of event type (r9)
-	 * N.B. #1: This is already endian safe (see ptrace.h)
-	 *      #2: Only r9 can be used as scratch as it is already clobbered
-	 *          and it's contents are no longer needed by the latter part
-	 *          of exception prologue
-	 */
-	lsl  r9, r8, 16
-	or   r9, r9, orig_r8_IS_SCALL
-
-	SAVE_ALL_EXCEPTION  r9
+	SAVE_ALL_EXCEPTION  orig_r8_IS_SCALL
 .endm
 
 /*--------------------------------------------------------------

arch/arc/include/asm/syscall.h

@@ -18,7 +18,7 @@ static inline long
 syscall_get_nr(struct task_struct *task, struct pt_regs *regs)
 {
 	if (user_mode(regs) && in_syscall(regs))
-		return regs->orig_r8;
+		return regs->r8;
 	else
 		return -1;
 }
@@ -26,8 +26,7 @@ syscall_get_nr(struct task_struct *task, struct pt_regs *regs)
 static inline void
 syscall_rollback(struct task_struct *task, struct pt_regs *regs)
 {
-	/* XXX: I can't fathom how pt_regs->r8 will be clobbered ? */
-	regs->r8 = regs->orig_r8;
+	regs->r0 = regs->orig_r0;
 }
 
 static inline long

arch/arc/include/uapi/asm/ptrace.h

@@ -44,7 +44,7 @@ struct user_regs_struct {
 		long r19, r18, r17, r16, r15, r14, r13;
 	} callee;
 	long efa;	/* break pt addr, for break points in delay slots */
-	long stop_pc;	/* give dbg stop_pc directly after checking orig_r8 */
+	long stop_pc;	/* give dbg stop_pc after ensuring brkpt trap */
 };
 #endif /* !__ASSEMBLY__ */