hle/kernel: Fix data race in GetCurrentHostThreadID

As reported by tsan, host_thread_ids could be read while
any of the RegisterHostThread variants were called.

To fix this, lock the register mutex when yuzu is running in multicore
mode and GetCurrentHostThreadID is called.

src/core/hle/kernel/kernel.cpp

@@ -219,6 +219,7 @@ struct KernelCore::Impl {
                 return static_cast<u32>(system.GetCpuManager().CurrentCore());
             }
         }
+        std::unique_lock lock{register_thread_mutex};
         const auto it = host_thread_ids.find(this_id);
         if (it == host_thread_ids.end()) {
             return Core::INVALID_HOST_THREAD_ID;
@@ -324,7 +325,7 @@ struct KernelCore::Impl {
     std::unordered_map<std::thread::id, u32> host_thread_ids;
     u32 registered_thread_ids{Core::Hardware::NUM_CPU_CORES};
     std::bitset<Core::Hardware::NUM_CPU_CORES> registered_core_threads;
-    std::mutex register_thread_mutex;
+    mutable std::mutex register_thread_mutex;
 
     // Kernel memory management
     std::unique_ptr<Memory::MemoryManager> memory_manager;