gdb: Add native support for ARC in GNU/Linux

With this patch in place it is possible to build a GDB that
can run on ARC (GNU/Linux) hosts for debugging ARC targets.

The "arc-linux-nat.c" is a rather small one that mostly deals
with registers and a few thread related hooks.

v2 [1]:
- Remove "void" from the input of "_initialize_arc_linux_nat ()"

[1] Tom's remark after the first patch
https://sourceware.org/pipermail/gdb-patches/2020-November/173223.html

gdb/ChangeLog:

	* Makefile.in (ALLDEPFILES): Add arc-linux-nat.c.
	* configure.host (host to gdb names): Add arc*-*-linux*.
	* configure.nat (gdb_host_cpu): Add arc.
	* arc-linux-nat.c: New.