ARC: Do not overwrite DEBUG.UB bit in arc_dbg_enter_debug

Force halt CPU by doing read-modify-write of DEBUG bit, instead of plain
write.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
ARC: Update JTAG layer to be compatible with latest OpenOCD

Function jtag_add_dr_out has been removed from OpenOCD so we cannot use it
anymore.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>