CPU: Save and restore the TPIDR_EL0 system register on every context switch.

Note that there's currently a dynarmic bug preventing this register from being written.

src/core/hle/kernel/scheduler.cpp

@@ -56,6 +56,8 @@ void Scheduler::SwitchContext(Thread* new_thread) {
     if (previous_thread) {
         previous_thread->last_running_ticks = CoreTiming::GetTicks();
         cpu_core->SaveContext(previous_thread->context);
+        // Save the TPIDR_EL0 system register in case it was modified.
+        previous_thread->tpidr_el0 = cpu_core->GetTPIDR_EL0();
 
         if (previous_thread->status == THREADSTATUS_RUNNING) {
             // This is only the case when a reschedule is triggered without the current thread
@@ -87,6 +89,7 @@ void Scheduler::SwitchContext(Thread* new_thread) {
 
         cpu_core->LoadContext(new_thread->context);
         cpu_core->SetTlsAddress(new_thread->GetTLSAddress());
+        cpu_core->SetTPIDR_EL0(new_thread->GetTPIDR_EL0());
         cpu_core->ClearExclusiveState();
     } else {
         current_thread = nullptr;

src/core/hle/kernel/thread.cpp

@@ -313,6 +313,7 @@ ResultVal<SharedPtr<Thread>> Thread::Create(std::string name, VAddr entry_point,
     thread->status = THREADSTATUS_DORMANT;
     thread->entry_point = entry_point;
     thread->stack_top = stack_top;
+    thread->tpidr_el0 = 0;
     thread->nominal_priority = thread->current_priority = priority;
     thread->last_running_ticks = CoreTiming::GetTicks();
     thread->processor_id = processor_id;

src/core/hle/kernel/thread.h

@@ -182,6 +182,14 @@ public:
         return tls_address;
     }
 
+    /*
+     * Returns the value of the TPIDR_EL0 Read/Write system register for this thread.
+     * @returns The value of the TPIDR_EL0 register.
+     */
+    u64 GetTPIDR_EL0() const {
+        return tpidr_el0;
+    }
+
     /*
      * Returns the address of the current thread's command buffer, located in the TLS.
      * @returns VAddr of the thread's command buffer.
@@ -213,6 +221,7 @@ public:
     s32 processor_id;
 
     VAddr tls_address; ///< Virtual address of the Thread Local Storage of the thread
+    u64 tpidr_el0;     ///< TPIDR_EL0 read/write system register.
 
     SharedPtr<Process> owner_process; ///< Process that owns this thread