ARC: pt_regs update #3: Remove unused gutter at start of callee_regs

This is trickier than prev two:

* context switching code saves kernel mode callee regs in the format of
  struct callee_regs thus needs adjustment. This also reduces the height
  of topmost kernel stack frame by 1 word.

* Since kernel stack unwinder is sensitive to height of topmost kernel
  stack frame, that needs a word of adjustment too.

ptrace needs a bit of updating since pt_regs now diverges from
user_regs_struct.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

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

@@ -38,8 +38,8 @@ struct user_regs_struct {
 		long r12, r11, r10, r9, r8, r7, r6, r5, r4, r3, r2, r1, r0;
 		long sp;
 	} scratch;
+	long pad2;
 	struct {
-		long pad;
 		long r25, r24, r23, r22, r21, r20;
 		long r19, r18, r17, r16, r15, r14, r13;
 	} callee;