[Zion] Add a semaphore primitive with related syscalls.

zion/object/semaphore.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include <glacier/container/intrusive_list.h>
+#include <glacier/memory/ref_ptr.h>
+
+#include "include/ztypes.h"
+#include "object/kernel_object.h"
+#include "object/thread.h"
+
+class Semaphore;
+
+template <>
+struct KernelObjectTag<Semaphore> {
+  static const uint64_t type = KernelObject::SEMAPHORE;
+};
+
+class Semaphore : public KernelObject {
+ public:
+  uint64_t TypeTag() override { return KernelObject::SEMAPHORE; }
+  static uint64_t DefaultPermissions() {
+    return kZionPerm_Wait | kZionPerm_Signal;
+  }
+
+  static glcr::RefPtr<Semaphore> Create();
+
+  void Wait();
+  void Signal();
+
+ private:
+  uint8_t lock_ = 0;
+
+  glcr::IntrusiveList<Thread> blocked_threads_;
+};