Add method for releasing a waiting interrupt (#2347)

Useful for a new high level interrupt implementation.

hal/src/main/native/athena/HALInternal.h

@@ -0,0 +1,15 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2020 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <stdint.h>
+
+namespace hal {
+void ReleaseFPGAInterrupt(int32_t interruptNumber);
+
+}  // namespace hal