hw: gate the iDMA tracer behind the DEBUG define

[DanielKellerM]

Follows up on #328 and coordinates with @colluca's upcoming TRACE change.

idma_inst64_top was instantiated with a hardcoded .DMATracing (1), so the iDMA backend wrote dma_trace_*.log on every run. Rather than couple this to DEBUG (which also pulls in +acc and slows the sim), it now follows the same TRACE opt-out being introduced for the snitch_cc trace: traces on by default, TRACE=OFF disables them.

Two commits:

• hw: gate the iDMA tracer with the TRACE_OFF opt-out - idma_inst64_top derives .DMATracing from a local DMATrace parameter gated by `ifndef TRACE_OFF, so the iDMA trace is silenced together with the snitch_cc trace under TRACE=OFF.
• make: wire the TRACE_OFF opt-out into the Verilator flow - the trace define reaches only the vsim bender flags; Verilator builds from the base flags. This adds the TRACE=OFF -> -DTRACE_OFF guard to SN_VLT_BENDER_FLAGS so the opt-out works under Verilator too.

Coordination with the TRACE change

DMATracing is an elaboration-time parameter, so TRACE_OFF has to be a compile define (bender -DTRACE_OFF / --vlog-arg), not just a +define on the vsim run command. SN_VSIM_FLAGS feeds the vsim simulate step (vsim.mk:103/110), while SN_VLOG_FLAGS/bender feed the compile step (vsim.mk:90), so a +define+TRACE_OFF on SN_VSIM_FLAGS alone would not reach either the snitch_cc `ifndef or this parameter. This PR handles the Verilator wiring; the vsim compile-side wiring belongs with the TRACE change.

Verified (iDMA side, self-contained on Verilator)

dma_1d on Verilator: default build -> iDMA dma_trace_*.log present; TRACE=OFF build -> absent. (The snitch_cc .dasm trace stays on the old DEBUG gate until the TRACE change lands, so it reads 0 in both cases here; it will follow TRACE_OFF once that lands.)

Draft until the TRACE change lands; will rebase and re-run the full on/off smoke-check on both flows then.

[Copilot]

Pull request overview

This PR ensures iDMA and Snitch instruction tracing are consistently controlled by a single DEBUG knob across both vsim and Verilator simulation flows, preventing unwanted log generation when DEBUG is off.

Changes:

• Gate iDMA tracing (DMATracing) behind `ifdef DEBUG by deriving it from a local DMATrace parameter.
• Wire -DDEBUG into the Verilator Bender flags when DEBUG=ON, aligning behavior with the vsim flow.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
make/verilator.mk	Adds conditional -DDEBUG to Verilator’s Bender flags when DEBUG=ON.
hw/snitch_cluster/src/snitch_cc.sv	Drives idma_inst64_top.DMATracing from a DEBUG-gated localparam instead of a hardcoded enable.

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