signals: preserve the longjmp return value on fallback
Pointer-mangling validation makes the direct longjmp fallback reachable on supported glibc targets. Hardcoding its return value converts preemption delivery into cancellation even though the caller requested a distinct reset code.
Pass the requested nonzero value through unchanged so simulated and libc-managed longjmps resume the reset point with identical semantics.
Co-authored-by: OpenAI Codex <noreply@openai.com>
