libuv-wasix: use WASIX posix_spawn/proc_join for child processes

[artemyarulin]

UPD: Not to be merged, but edgejs will look to this PR branch as a quick way to resolve stalling wasmerio/edgejs#34

---

Replace the broken WASIX fork fallback with a real spawn backend.

• add a WASIX-specific posix_spawn path in process.c
• map stdio/cwd/path handling onto WASIX spawn file actions
• reap child exit via proc_join instead of waitpid
• poll process handles from the event loop on WASIX
• fix WASIX pipe setup for stdin/stdout/stderr semantics

This makes child_process spawning work through the WASIX process model instead of reporting fake fork success and hanging.

[Arshia001]

Closing as duplicate of #7.

#7 is a clean redo from the ea792e2 disable-fork baseline with POSIX-boundary-only spawn/reap (posix_spawnp + libc waitpid(WNOHANG), no direct __wasi_* in libuv). EdgeJS verification for the PR libuv#34 stall repros passes against d8ebea9 on fix/spawn-posix-v2.