ARC: pt_regs cleanup #4: no need to keep backup of r8 (RIP orig_r8)

ARC: pt_regs cleanup #4: no need to keep backup of r8 (RIP orig_r8)

Historically, pt_regs have had orig_r8, an overloaded container for
  (1) backup copy of syscall number (in case of syscall 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 after having been updated in-place with syscall ret value).

So remove stuffing of syscall num in orig_r8 - simplifies SAVE_ALL_TRAP
Use it exclusively for (2) hence rename to @event - RIP orig_r8

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