Use fmt::localtime instead of thread-unsafe std::localtime

fmt::localtime is also less awkward to use compared to std::localtime.

Source/Core/AudioCommon/AudioCommon.cpp

@@ -210,7 +210,7 @@ void StartAudioDump()
   std::string path_prefix = File::GetUserPath(D_DUMPAUDIO_IDX) + SConfig::GetInstance().GetGameID();
 
   std::string base_name =
-      fmt::format("{}_{:%Y-%m-%d_%H-%M-%S}", path_prefix, *std::localtime(&start_time));
+      fmt::format("{}_{:%Y-%m-%d_%H-%M-%S}", path_prefix, fmt::localtime(start_time));
 
   const std::string audio_file_name_dtk = fmt::format("{}_dtkdump.wav", base_name);
   const std::string audio_file_name_dsp = fmt::format("{}_dspdump.wav", base_name);

Source/Core/Common/SettingsHandler.cpp

@@ -140,6 +140,6 @@ std::string SettingsHandler::GenerateSerialNumber()
   // Must be 9 characters at most; otherwise the serial number will be rejected by SDK libraries,
   // as there is a check to ensure the string length is strictly lower than 10.
   // 3 for %j, 2 for %H, 2 for %M, 2 for %S.
-  return fmt::format("{:%j%H%M%S}", *std::localtime(&t));
+  return fmt::format("{:%j%H%M%S}", fmt::localtime(t));
 }
 }  // namespace Common

Source/Core/Core/Core.cpp

@@ -739,7 +739,7 @@ static std::string GenerateScreenshotName()
 
   const std::time_t cur_time = std::time(nullptr);
   const std::string base_name =
-      fmt::format("{}_{:%Y-%m-%d_%H-%M-%S}", path_prefix, *std::localtime(&cur_time));
+      fmt::format("{}_{:%Y-%m-%d_%H-%M-%S}", path_prefix, fmt::localtime(cur_time));
 
   // First try a filename without any suffixes, if already exists then append increasing numbers
   std::string name = fmt::format("{}.png", base_name);

Source/Core/VideoCommon/FrameDump.cpp

@@ -86,7 +86,7 @@ std::string GetDumpPath(const std::string& extension, std::time_t time, u32 inde
       File::GetUserPath(D_DUMPFRAMES_IDX) + SConfig::GetInstance().GetGameID();
 
   const std::string base_name =
-      fmt::format("{}_{:%Y-%m-%d_%H-%M-%S}_{}", path_prefix, *std::localtime(&time), index);
+      fmt::format("{}_{:%Y-%m-%d_%H-%M-%S}_{}", path_prefix, fmt::localtime(time), index);
 
   const std::string path = fmt::format("{}.{}", base_name, extension);