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/kernel/asm-offsets.c

@@ -60,5 +60,6 @@ int main(void)
 	DEFINE(PT_r6, offsetof(struct pt_regs, r6));
 	DEFINE(PT_r7, offsetof(struct pt_regs, r7));
 
+	DEFINE(SZ_CALLEE_REGS, sizeof(struct callee_regs));
 	return 0;
 }