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

ARC: pt_regs cleanup #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.

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