Make the register handling match GDB client, and prepare for ARC ABI v3.

2013-03-04  Jeremy Bennett <jeremy.bennett@embecosm.com>

	* linux-arc-low.c (orig_r8) Deleted.
	(arc_cannot_store_register, arc_cannot_fetch_register): Updated to
	correctly reflect user space r/w status of registers.
	(arc_set_pc): Simplified - issues are dealt with in
	arc_fill_gregset and arc_store_gregset.
	(arc_fill_gregset, arc_store_gregset): No need for orig_r8 any
	more (it goes in ARV v3 ABI anyway).