runtime: panic FP walk can start from a returned C helper frame

[cpunion]

Summary

An unrecovered panic can fall back from LLGo's Go-style traceback to the raw clite PC dump when the physical frame-pointer walk starts from an invalid frame. The failure was first exposed by combining a logging dependency graph with a panic probe, but the minimized reproducer only needs a shallow panic, the public runtime package, DWARF, and -O0.

This is independent of #2115: funcinfo and runtime.Stack remain available; only the panic-time physical walk returns no frames.

Reproduction

On Darwin/arm64 with current main:

package main

import "runtime"

//go:noinline
func panicSite() {
	panic("shallow-panic")
}

func main() {
	_ = runtime.NumCPU()
	panicSite()
}

llgo run -a -O0 -ldflags=-w=false main.go

Actual output falls back to raw addresses:

panic: shallow-panic

[0x... command-line-arguments.panicSite+0x20, SP = ...]
[0x... command-line-arguments.main+..., SP = ...]
[0x... main+..., SP = ...]

Expected output starts with goroutine 1 [running]: and contains Go-style main.panicSite(...) and main.main(...) frames.

Root cause

runtime/internal/lib/runtime/_wrap/runtime.c:llgo_framepointer returns __builtin_frame_address(0), the C helper's own frame address. The Go walker dereferences that address after the helper has returned, so it is a dangling stack-frame pointer. In the failing O0/DWARF layout its saved-FP slot has already been overwritten (observed value: 64), and the monotonic-chain guard stops immediately.

Optimized layouts often leave the stale bytes intact long enough to hide the bug. This explains why #2143's O2 pipeline appears to fix the larger probe while -O0 still fails.

The helper should read its saved caller FP while its own frame is alive and return that FP value. That matches the Go-side walker's existing assumption and skip++ accounting.

Acceptance

• A shallow unrecovered panic emits a Go-style traceback under -O0 -ldflags=-w=false.
• Caller/Callers/Stack and panic traceback tests pass at O0/O2 with and without DWARF.
• Darwin and Linux native builds pass.

Proposed fix

• runtime: start FP walks from a live caller frame #2151 returns the saved caller FP, removes the obsolete skip compensation, and adds O0/O2 × DWARF on/off regression coverage.