[util] Add thread id to kernel spinlocks

Expose a function __current_thread_id() and use it to record the thread
id on a spinlock waiter when called from the kernel.

src/kernel/objects/thread.cpp

@@ -180,3 +180,10 @@ thread::create_idle_thread(process &kernel, uintptr_t rsp0)
 }
 
 } // namespace obj
+
+uint32_t
+__current_thread_id()
+{
+    cpu_data &cpu = current_cpu();
+    return cpu.thread ? cpu.thread->obj_id() : -1u;
+}

src/kernel/objects/thread.h

@@ -207,3 +207,5 @@ private:
 };
 
 } // namespace obj
+
+extern "C" uint32_t __current_thread_id();