ARCv2 Core3 cpus are comming with dbnz support. Add this feature on
the tune option.

gcc/
2018-01-26  Claudiu Zissulescu  <claziss@synopsys.com>

        * config/arc/arc-arch.h (arc_tune_attr): Add ARC_TUNE_CORE_3.
        * config/arc/arc.c (arc_sched_issue_rate): Use ARC_TUNE_... .
        (arc_init): Likewise.
        (arc_override_options): Likewise.
        (arc_file_start): Choose Tag_ARC_CPU_variation based on arc_tune
        value.
        (hwloop_fail): Use TARGET_DBNZ when we want to check for dbnz insn
        support.
        * config/arc/arc.h (TARGET_DBNZ): Define.
        * config/arc/arc.md (attr tune): Add core_3, use ARC_TUNE_... to
        properly set the tune attribute.
        (dbnz): Use TARGET_DBNZ guard.
        * config/arc/arc.opt (mtune): Add core3 option.

From-SVN: r257085