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

[cpunion]

Depends on #2090

Problem

LLGo implements panic/recover with longjmp. The jump removes 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 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 runtime: store goroutine state behind pthread-key getg #2090's pthread-key g;
• use the same getg path for ordinary panic and hardware-fault snapshots;
• 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 snapshot state adds no pthread key and no second lifetime mechanism. The LLGo integration test keeps two goroutines alive concurrently and verifies that PCs, fault flags, and recover-frame marks remain isolated from each other and from main.

This PR has an independent diff of +708/-109 across 16 files.

Conformance

Removed xfails for:

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

Validation

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

• macOS arm64, Go 1.26.5: compiler PC-line tests, snapshot isolation, and three sequential recovered C faults pass.
• Ubuntu arm64, Go 1.26.5, LLVM 19.1.7: the same isolation and C-fault paths pass in an 8 GiB, 2 CPU, no-swap, 512-PID container.
• runtime/internal/runtime compiles under the host Go toolchain.
• No existing test is skipped or ignored.

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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