feat(syscall): add clone3 support

[Mivik]

Description

Building upon #79, this PR:

• Fixed compilation error
• Added some clone3-specific checks
• Implemented thread-specific pidfd support, ensuring correct pidfd is returned when clone is invoked with the CLONE_THREAD | CLONE_PIDFD flags.

[Copilot]

Pull request overview

This PR extends StarryOS’s task/syscall layer to support the clone3 syscall and improves pidfd handling (including a thread-specific pidfd path for CLONE_THREAD | CLONE_PIDFD), plus related exit notification plumbing.

Changes:

• Added sys_clone3 implementation and wired it into syscall dispatch.
• Refactored clone implementation around a unified CloneArgs/CloneFlags path and added thread-specific pidfd creation.
• Introduced a per-thread exit PollSet and expanded pidfd_open/PidFd to support thread pidfds and nonblocking mode.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
kernel/src/task/ops.rs	Wakes a per-thread exit event during thread exit.
kernel/src/task/mod.rs	Adds exit_event to Thread for exit notification.
kernel/src/syscall/task/mod.rs	Registers and re-exports the new clone3 module.
kernel/src/syscall/task/clone3.rs	Implements sys_clone3 argument parsing and dispatch into unified clone logic.
kernel/src/syscall/task/clone.rs	Refactors clone into CloneArgs::do_clone, adds new clone flags, and adds thread/process pidfd creation.
kernel/src/syscall/mod.rs	Adds syscall dispatch case for Sysno::clone3.
kernel/src/syscall/fs/pidfd.rs	Adds pidfd_open flags parsing (thread + nonblock) and thread pidfd creation.
kernel/src/file/pidfd.rs	Adds thread/process pidfd constructors, nonblocking state, and changes polling/exit tracking mechanism.
build-dora.sh	Adds a local build/deploy helper script for a specific environment.

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