fix: run the backend at -Oless for unoptimized builds

[ghaith]

Companion to #1819 for release/1.0.x (stacked on it; retargets automatically when #1819 merges). Together they close the string-temp stack-exhaustion problem for unoptimized builds.

Problem

#1819 brackets lowered call temporaries with lifetime markers so LLVM's stack coloring can overlap their slots — but stack coloring is a machine pass that only runs when the backend optimizes. At -Onone the markers are inert, and string-heavy POU bodies keep one dead STRING[2048] slot per call site: a representative function block needs ~112 KB of frame and overflows constrained task stacks at entry.

Fix

Decouple the two optimization dials. -Onone keeps the IR pipeline at default<O0> — no inlining, no unrolling, no IR-level transforms that damage source-level debugging — but creates the TargetMachine at codegen level Less, so the machine pipeline (instruction selection via SelectionDAG, greedy register allocation, and crucially stack coloring) runs.

Frame size of the representative function block at -Onone: 111,760 B → 10,312 B.

Debug-experience validation

Compared gdb behavior on a multi-file test program (small inlinable functions, cross-file calls, unrollable and data-dependent loops, string-taking and string-returning functions) at -Onone before/after, plus -Oless/-Odefault/-Oaggressive for context:

• Function and line breakpoints bind and hit identically; loop-body line breakpoints still hit once per iteration with the induction variable readable at each stop.
• Backtraces, info args/info locals, and finish behave identically; zero <optimized out> locals (those appear starting at -Oless, caused by the IR pipeline, which stays off here).
• Line-stepping through loops actually improves: the FastISel-era quirk where next repeated the loop-header line and never showed the body is gone with SelectionDAG.
• One artifact: reading a local before its first assignment can show stack garbage where it previously happened to read the zero-initialized slot. Initialized values are unaffected.

Testing

Full unit suite, snapshot tests (no drift) and the lit end-to-end suite pass. Frame measurement above from objdump on the compiled object.

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