gdb: Add default reggroups for ARC

There is no reggroups set in ARC.  If a "maintenance print reggroups"
command is issued, the default register set is dumped (which is fine).

However, if a new group is added via an XML file, then that will
become the _only_ group.  This behavior causes gdb.xml/tdesc-regs.exp
to fail.

Fixes gdb.xml/tdesc-regs.exp on ARC.

gdb/ChangeLog:

	* arc-tdep.c (arc_add_reggroups): New function.
	(arc_gdbarch_init): Call arc_add_reggroups.