State: Make use of re-entrant variant of localtime

Makes this member function thread-safe. We can also unify the string
handling to make it easier to maintain.

Source/Core/Core/State.cpp

@@ -7,6 +7,7 @@
 #include <atomic>
 #include <condition_variable>
 #include <filesystem>
+#include <locale>
 #include <map>
 #include <memory>
 #include <mutex>
@@ -15,6 +16,7 @@
 #include <utility>
 #include <vector>
 
+#include <fmt/chrono.h>
 #include <fmt/format.h>
 
 #include <lz4.h>
@@ -278,21 +280,11 @@ static double GetSystemTimeAsDouble()
 static std::string SystemTimeAsDoubleToString(double time)
 {
   // revert adjustments from GetSystemTimeAsDouble() to get a normal Unix timestamp again
-  time_t seconds = static_cast<time_t>(time) + DOUBLE_TIME_OFFSET;
-  errno = 0;
-  tm* local_time = localtime(&seconds);
-  if (errno != 0 || !local_time)
-    return "";
+  const time_t seconds = static_cast<time_t>(time) + DOUBLE_TIME_OFFSET;
+  const tm local_time = fmt::localtime(seconds);
 
-#ifdef _WIN32
-  wchar_t tmp[32] = {};
-  wcsftime(tmp, std::size(tmp), L"%x %X", local_time);
-  return WStringToUTF8(tmp);
-#else
-  char tmp[32] = {};
-  strftime(tmp, sizeof(tmp), "%x %X", local_time);
-  return tmp;
-#endif
+  // fmt is locale agnostic by default, so explicitly use current locale.
+  return fmt::format(std::locale{""}, "{:%x %X}", local_time);
 }
 
 static std::string MakeStateFilename(int number);