Fix MinGW stat redefinition and thread-safe localtime

- Guard #define stat _stat with !defined(__MINGW32__) to avoid conflicts with MinGW's own POSIX-compliant stat implementation

- Replace _MSC_VER check with defined(_WIN32) for localtime_s to allow all Windows compilers including MinGW to use the secure function

Fixes #3038, #3037

src/user/user_vfs.cc

@@ -15,7 +15,7 @@
 #include "user/user_vfs.h"
 
 #include <sys/stat.h>
-#ifdef _WIN32
+#if defined(_WIN32) && !defined(__MINGW32__)
 #define stat _stat
 #endif