[hal] Initial SystemCore empty HAL (#7454)

hal/src/main/native/systemcore/Constants.cpp

@@ -0,0 +1,19 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include "hal/Constants.h"
+
+#include "ConstantsInternal.h"
+
+using namespace hal;
+
+namespace hal::init {
+void InitializeConstants() {}
+}  // namespace hal::init
+
+extern "C" {
+int32_t HAL_GetSystemClockTicksPerMicrosecond(void) {
+  return kSystemClockTicksPerMicrosecond;
+}
+}  // extern "C"