ARC: gdbserver breakage in Big-Endian configuration #2

[Based on mainline commit 352c1d95e3220d0: "ARC: stop using
pt_regs->orig_r8"]

Stop using orig_r8 as it could get clobbered by ST in trap_with_param,
and further it is semantically not needed either.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>