[Based on #2090] runtime,cl: preserve panic-site frames across defer and recover

[cpunion]

Depends on #2090

Dependency

Stack: #2079 -> #2090 -> this PR.

#2079 provides //llgo:tls and //llgo:gls; #2090 stores goroutine runtime state in one GLS-backed g. This PR uses the same facilities and no longer adds a pthread key.

Problem

LLGo implements panic/recover with longjmp. The jump removes the panicking frames before deferred functions run, while gc keeps those logical frames visible to runtime.Caller, runtime.Callers/CallersFrames, and debug.Stack, including after recover until the recovering defer returns.

#2028 already owns recoverable hardware faults, the SA_SIGINFO handler, and fault-context unwinding. The missing layer is preserving ordinary panic snapshots and exposing both ordinary and fault snapshots through caller APIs.

Implementation

• Capture the physical PC chain before longjmp and splice it below live deferred frames.
• Store the fixed, pointer-free panicPCStore in //llgo:tls. Signal handling performs no allocation, pthread-key lookup, or first-use initialization.
• Use an ordinary global fallback for host builds, bare-metal, and Nintendo Switch builds.
• Use [Based on #2079] runtime: store goroutine state in //llgo:gls #2090's g state to determine whether a panic is still active.
• Keep a recovered snapshot observable only while the recovering deferred frame is live.
• Attribute deferred execution to the function closing brace and emit an explicit PC/line anchor for panic.
• Compile C with frame pointers so fault chains can cross C frames.

The Linux validation found that the old frame liveness check inspected a C helper frame after it had returned; the next call could reuse that slot and terminate the walk. The final implementation synchronously consumes the existing helper's active FP links in Go and returns the stable Go caller frame. It adds no C symbol or cache ABI.

Conformance

Removed xfails for:

• fixedbugs/issue14646.go
• fixedbugs/issue5856.go
• fixedbugs/issue33724.go

The remaining statement-line cases are still limited by line granularity in untracked functions and are not skipped by this PR.

Validation

Tests were serialized with GOMAXPROCS=2, GOMEMLIMIT=8GiB, and -p=1 where supported.

• macOS arm64, Go 1.26.5: compiler PC-line tests, crosscompile tests, caller acceptance, three sequential recovered C faults, and the three focused GOROOT cases pass. Peak RSS for caller acceptance was 1.61 GiB.
• Ubuntu arm64, Go 1.26.5, LLVM 19.1.7: the same affected compiler paths, recovered C-fault regression, and focused GOROOT cases pass in a container hard-limited to 15 GiB, 2 CPU, and 512 PIDs.
• The root coverage run executes every new cl/compile.go branch; deferRunPos is 100% covered. The nested runtime module is exercised by LLGo integration tests because the root Codecov job does not instrument it.
• runtime/internal/runtime and runtime/internal/lib/runtime compile under the host Go toolchain.

Limits

• Statement-level fault lines in untracked functions still need the prebuilt PC-line follow-up.
• Linux cannot reliably name non-dynamic C symbols; captured PCs are retained, but a C frame can be attributed to a neighboring symbol.
• Alternate signal stacks remain outside this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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