arc: Add ARCv2 XML target along with refactoring

arc: Add ARCv2 XML target along with refactoring

A few changes have been made to make the register support simpler,
more flexible and extendible.  The trigger for most of these changes
are the remarks [1] made earlier for v2 of this patch.  The noticeable
improvements are:

- The arc XML target features are placed under gdb/features/arc
- There are two cores (based on ISA) and one auxiliary feature:
  v1-core: ARC600, ARC601, ARC700
  v2-core: ARC EM, ARC HS
  aux: common in both
- The XML target features represent a minimalistic sane set of
  registers irrespective of application (baremetal or linux).
- A concept of "feature" class has been introduced in the code.
  The "feature" object is constructed from BFD and GDBARCH data.
  It contains necessary information (ISA and register size) to
  determine which XML target feature to use.
- A new structure (ARC_REGISTER_FEATURE) is added that allows
  providing index, names, and the necessity of registers. This
  simplifies the sanity checks and future extendibility.
- Documnetation has been updated to reflect ARC features better.
- Although the feature names has changed, there still exists
  backward compatibility with older names through
  find_obsolete_[core,aux]_names() functions.

The last two points were inspired from RiscV port.

[1]
https://sourceware.org/pipermail/gdb-patches/2020-May/168511.html

gdb/ChangeLog:

	* arch/arc.h
	  (arc_gdbarch_features): New class to stir the selection of target XML.
	  (arc_create_target_description): Use FEATURES to choose XML target.
	  (arc_lookup_target_description): Use arc_create_target_description
	  to create _new_ target descriptions or return the already created
	  ones if the FEATURES is the same.
	* arch/arc.c: Implementation of prototypes described above.
	* gdb/arc-tdep.h (arc_regnum enum): Add more registers.
	  (arc_gdbarch_features_init): Initialize the FEATURES struct.
	* arc-tdep.c (*_feature_name): Make feature names consistent.
	  (arc_register_feature): A new struct to hold information about
	  registers of a particular target/feature.
	  (arc_check_tdesc_feature): Check if XML provides registers in
	  compliance with ARC_REGISTER_FEATURE structs.
	  (arc_update_acc_reg_names): Add aliases for r58 and r59.
	  (determine_*_reg_feature_set): Which feature name to look for.
	  (arc_gdbarch_features_init): Given MACH and ABFD, initialize FEATURES.
	  (mach_type_to_arc_isa): Convert from a set of binutils machine types
	  to expected ISA enums to be used in arc_gdbarch_features structs.
	* features/Makefile (FEATURE_XMLFILES): Add new files.
	* gdb/features/arc/v1-aux.c: New file.
	* gdb/features/arc/v1-aux.xml: Likewise.
	* gdb/features/arc/v1-core.c: Likewise.
	* gdb/features/arc/v1-core.xml: Likewise.
	* gdb/features/arc/v2-aux.c: Likewise.
	* gdb/features/arc/v2-aux.xml: Likewise.
	* gdb/features/arc/v2-core.c: Likewise.
	* gdb/features/arc/v2-core.xml: Likewise.
	* NEWS (Changes since GDB 9): Announce obsolence of old feature names.

gdb/doc/ChangeLog:

	* gdb.texinfo (Synopsys ARC): Update the documentation for ARC
	Features.

gdb/testsuite/ChangeLog:

	* gdb.arch/arc-tdesc-cpu.xml: Use new feature names.