arc: Align internal regnums with architectural regnums

arc: Align internal regnums with architectural regnums

Add ARC_LIMM_REGNUM to arc_regnum enumeration and assign a number 62 to it.
This ensures that for core registers internal register numbers in this enum are
the same as architectural numbers.  This allows to use internal register
numbers in the contexts where architectural number is implied, for example when
disassembling instruction during prologue analysis.

gdb/ChangeLog:

yyyy-mm-dd  Anton Kolesov  <anton.kolesov@synopsys.com>

	* arc-tdep.c (core_v2_register_names, core_arcompact_register_names)
	Add "limm" and "reserved".
	(arc_cannot_fetch_register, arc_cannot_store_register): Add
	ARC_RESERVED_REGNUM and ARC_LIMM_REGNUM.
	* arc-tdep.h (arc_regnum): Likewise.