thread/spinlock: add move assignment

thread/spinlock.cpp

@@ -27,6 +27,21 @@ spinlock::spinlock (spinlock &&rhs) noexcept:
 { ; }
 
 
+//-----------------------------------------------------------------------------
+spinlock& spinlock::operator= (spinlock &&rhs) noexcept
+{
+    lock ();
+    rhs.lock ();
+
+    held = rhs.held.load ();
+
+    rhs.unlock ();
+    unlock ();
+
+    return *this;
+}
+
+
 ///////////////////////////////////////////////////////////////////////////////
 void
 spinlock::lock (void)