core_timing: Make use of uintptr_t to represent user_data

Makes the interface future-proofed for supporting other platforms in the event we ever support platforms with differing pointer sizes. This way, we have a type in place that is always guaranteed to be able to represent a pointer exactly.

src/core/hle/kernel/kernel.cpp

@@ -145,7 +145,7 @@ struct KernelCore::Impl {
 
     void InitializePreemption(KernelCore& kernel) {
         preemption_event = Core::Timing::CreateEvent(
-            "PreemptionCallback", [this, &kernel](u64, std::chrono::nanoseconds) {
+            "PreemptionCallback", [this, &kernel](std::uintptr_t, std::chrono::nanoseconds) {
                 {
                     SchedulerLock lock(kernel);
                     global_scheduler.PreemptThreads();