Should use `_fstat32()` on 32bit Windows.
MODIFY
mrbgems/mruby-io/src/file.c
mrbgems/mruby-io/src/file.c
typedef struct stat mrb_stat;
# define mrb_stat(path, sb) stat(path, sb)
# define mrb_fstat(fd, sb) fstat(fd, sb)
+#elif defined MRB_INT32
+typedef struct __stat32 mrb_stat;
+# define mrb_stat(path, sb) _stat32(path, sb)
+# define mrb_fstat(fd, sb) _fstat32(fd, sb)
#else
typedef struct __stat64 mrb_stat;
# define mrb_stat(path, sb) _stat64(path, sb)
@@ -393,7 +397,7 @@ mrb_file_mtime(mrb_state *mrb, mrb_value self)
int fd = mrb_io_fileno(mrb, self);
struct stat st;
- if (fstat(fd, &st) == -1)
+ if (mrb_fstat(fd, &st) == -1)
return mrb_false_value();
return mrb_fixnum_value((mrb_int)st.st_mtime);
}

Should use `_fstat32()` on 32bit Windows.