[hal] Set HAL Notifier thread as RT by default (#3482)

This PR gives the Notifier HAL thread RT priority 40 in RobotBase after
HAL initialization and before the user code is run. This drastically
improves scheduling jitter for TimedRobot's AddPeriodic() functions (in
3512's experience).

It's too risky to set user code as RT because badly behaved code
will lock up the Rio (potentially requiring safe mode to recover).

This needs the user program to be setuid admin to succeed.

wpilibc/src/main/native/cppcs/RobotBase.cpp

@@ -20,6 +20,7 @@
 #include "WPILibVersion.h"
 #include "frc/DriverStation.h"
 #include "frc/Errors.h"
+#include "frc/Notifier.h"
 #include "frc/RobotState.h"
 #include "frc/livewindow/LiveWindow.h"
 #include "frc/smartdashboard/SmartDashboard.h"
@@ -35,6 +36,12 @@ int frc::RunHALInitialization() {
   }
   HAL_Report(HALUsageReporting::kResourceType_Language,
              HALUsageReporting::kLanguage_CPlusPlus, 0, GetWPILibVersion());
+
+  if (!frc::Notifier::SetHALThreadPriority(true, 40)) {
+    FRC_ReportError(warn::Warning, "{}",
+                    "Setting HAL Notifier RT priority to 40 failed\n");
+  }
+
   std::puts("\n********** Robot program starting **********");
   return 0;
 }