Restore SuspendThread/SetThreadContext preemption for true async preemption on Windows

Reverts from cooperative atomic-flag approach back to the full
SuspendThread/GetThreadContext/SetThreadContext/ResumeThread approach,
which can preempt pure CPU-bound loops (like `loop {}`) on Windows.

Changes:
1. Restored assembly stubs and thread context manipulation in monitor.rs
2. Removed check_preempt() cooperative approach from impl_facade! macro
3. Made coroutine_preemptive test run on all platforms (not Unix-only)
4. Removed Go-related comments per maintainer request

Agent-Logs-Url: https://github.com/acl-dev/open-coroutine/sessions/2348f69b-81c0-4cf0-9c52-9079bf4b243c

Co-authored-by: loongs-zhang <38336731+loongs-zhang@users.noreply.github.com>

Author: Copilot