#6539: don't divide caml_major_heap_increment by eight

Major heap increment is now stored in words or percent:

```c
extern uintnat caml_major_heap_increment; /* percent or words; see major_gc.c */
```

However, in caml_gc_get, the major heap increment is still divided by 8:

```c
Store_field (res, 1,Val_long(Wsize_bsize(caml_major_heap_increment)));/* i */
```

This leads to this behaviour:

```ocaml
let print x = print_int x; print_newline ()

let control = Gc.get ()
let () = print control.Gc.major_heap_increment

let () = Gc.set control
let () = print (Gc.get ()).Gc.major_heap_increment

let () = Gc.set { control with major_heap_increment = 40960 }
let () = print (Gc.get ()).Gc.major_heap_increment
```
Output:

    $ ./temp
    1
    0
    5120