Use wpi::mutex instead of std::mutex. (#730)

This uses a priority-aware mutex on Linux platforms.

Fixes #729.

hal/src/main/native/athena/Interrupts.cpp

@@ -32,7 +32,7 @@ struct Interrupt {
 class InterruptThread : public wpi::SafeThread {
  public:
   void Main() {
-    std::unique_lock<std::mutex> lock(m_mutex);
+    std::unique_lock<wpi::mutex> lock(m_mutex);
     while (m_active) {
       m_cond.wait(lock, [&] { return !m_active || m_notify; });
       if (!m_active) break;