Query multiple meta values at the same time :

Question

You’ll need to change your search form, PHP uses fieldname[] notation to accept multiple values for a parameter name.

<input type="checkbox" name="modele_du_processeur[]" value="Core i3" />
<input type="checkbox" name="modele_du_processeur[]" value="Core i5" />
<input type="checkbox" name="modele_du_processeur[]" value="Core i7" />

Once that’s done, you will get an array in $_GET["modele_du_processeur"], so you can work with

$args = array(
    'post_type' => 'pc-portables-gamer',
    'posts_per_page' => -1,
    'meta_query' => array(
        'relation' => 'AND',
        array( 'key' => 'modele_du_processeur', 'value' => $_GET['modele_du_processeur'] , 'compare' => 'IN' )
    )
);

$query = new WP_Query($args);