arc: Add evaluation of long jump targets

arc: Add evaluation of long jump targets

Standard get_longjmp_target implementation, similar to what is in arm-tdep.c.
Actual value of jb_pc should be set in init_osabi methods of particular OS/ABI
implementations.

gdb/ChangeLog:

	* arc-tdep.h (struct gdbarch_tdep) <jb_pc>: New field.
	* arc-tdep.c (arc_get_longjmp_target): New function.
	(arc_gdbarch_init): Set get_longjmp_target if jb_pc is non-negative.
	(arc_dump_tdep): Print jb_pc.