extmod/vfs: Support larger integer range in VFS stat time fields.

On ports like unix where the Epoch is 1970/1/1 and atime/mtime/ctime are in
seconds since the Epoch, this value will overflow a small-int on 32-bit
systems.  So far this is only an issue on 32-bit unix builds that use the
VFS layer (eg dev and coverage unix variants) but the fix (using
mp_obj_new_int_from_uint instead of MP_OBJ_NEW_SMALL_INT) is there for all
ports so as to not complicate the code, and because they will need the
range one day.

Also apply a similar fix to other fields in VfsPosix.stat because they may
also be large.

Signed-off-by: Damien George <damien@micropython.org>

extmod/vfs_fat.c

@@ -326,9 +326,9 @@ STATIC mp_obj_t fat_vfs_stat(mp_obj_t vfs_in, mp_obj_t path_in) {
     t->items[4] = MP_OBJ_NEW_SMALL_INT(0); // st_uid
     t->items[5] = MP_OBJ_NEW_SMALL_INT(0); // st_gid
     t->items[6] = mp_obj_new_int_from_uint(fno.fsize); // st_size
-    t->items[7] = MP_OBJ_NEW_SMALL_INT(seconds); // st_atime
-    t->items[8] = MP_OBJ_NEW_SMALL_INT(seconds); // st_mtime
-    t->items[9] = MP_OBJ_NEW_SMALL_INT(seconds); // st_ctime
+    t->items[7] = mp_obj_new_int_from_uint(seconds); // st_atime
+    t->items[8] = mp_obj_new_int_from_uint(seconds); // st_mtime
+    t->items[9] = mp_obj_new_int_from_uint(seconds); // st_ctime
 
     return MP_OBJ_FROM_PTR(t);
 }