[hal] Add high level way of getting systemserver NT instance (#7683)

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

@@ -2,8 +2,9 @@
 // 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 "SystemServer.h"
+#include "hal/SystemServer.h"
 
+#include "SystemServerInternal.h"
 #include "mrc/NtNetComm.h"
 
 namespace hal {
@@ -33,3 +34,10 @@ nt::NetworkTableInstance GetSystemServer() {
 }
 
 }  // namespace hal
+
+extern "C" {
+
+NT_Inst HAL_GetSystemServerHandle(void) {
+  return hal::ServerInstance.GetHandle();
+}
+}  // extern "C"