arc: Add support for Linux coredump files

With the implemenations in this patch, ARC gdb can handle
coredump related matters.  The binutils counter part of
this patch has already been pushed [1].

v2 [2]:
- arc_linux_collect_gregset: Use "reg <= ARC_LAST_REGNUM" instead of
  "reg < ARC_LAST_REGNUM" for the condition check of the for-loop.
- arc-linux-tdep.c: Use "ARC_LAST_REGNUM < ARRAY_SIZE (...)" instead of
  "ARC_LAST_REGNUM <= ARRAY_SIZE (...)" for the "asserts".
- Use "buf + arc_linux_core_reg_offsets[ARC_ERET_REGNUM]" instead of
  "buf + REG_OFF (6)".
- Fix a few typos/indentation.

v3 [3]:
- Use gdb_assert_not_reached(text) instead of gdb_assert (!text).
- Remove unnecessary braces in the for loop.

[1] arc: Add support for ARC HS extra registers in core files
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=2745674244d6aecddcf636475034bdb9c0a6b4a0

[2] First remarks
https://sourceware.org/pipermail/gdb-patches/2020-September/171912.html

[3] Second remarks
https://sourceware.org/pipermail/gdb-patches/2020-October/172302.html

gdb/ChangeLog:

	* arc-linux-tdep.h: New file.
	* arc-linux-tdep.c (arc_linux_core_reg_offsets,
	arc_linux_supply_gregset, arc_linux_supply_v2_regset,
	arc_linux_collect_gregset, arc_linux_collect_v2_regset,
	arc_linux_gregset, arc_linux_v2_regset,
	arc_linux_iterate_over_regset_sections,
	arc_linux_core_read_description): Implement.
	(arc_linux_init_osabi): Set iterate_over_regset_sections.
	* arc-tdep.h (ARC_OFFSET_NO_REGISTER): Declare.
        (arc_gdbarch_features_create): Add.
	* arc-tdep.c (arc_gdbarch_features_create): Not static anymore.

gdb/arc-tdep.c

@@ -1883,10 +1883,9 @@ mach_type_to_arc_isa (const unsigned long mach)
     }
 }
 
-/* Common construction code for ARC_ARCH_FEATURES struct.  If there
-   is no ABFD, then a FEATURE with default values is returned.  */
+/* See arc-tdep.h.  */
 
-static arc_arch_features
+arc_arch_features
 arc_arch_features_create (const bfd *abfd, const unsigned long mach)
 {
   /* Use 4 as a fallback value.  */