arc: Determine a branch target of DBNZ correctly

arc: Determine a branch target of DBNZ correctly

DBNZ instruction was moved from BRANCH class to a separate one - DBNZ.
Thus, it must be processed separately in arc_insn_get_branch_target
to correctly determine an offset for a possible branch.

The testsuite for DBNZ instruction verifies these cases:

     1. Check that dbnz does not branch and falls through if its source
        register is 0 after decrementing. GDB must successfully break
        on the following instruction after stepping over.
     2. Check that dbnz branches to the target correctly if its source register
        is not 0 after decrementing - GDB must successfully break on the target
        instruction if a forward branch is performed after stepping over.
     3. The same as point 2 but for a backward branching case.

Signed-off-by: Yuriy Kolerov <kolerov93@gmail.com>