expander: fix variable-reference-constant? on primitive

When setting up the namespaces that imitate primitive instances,
the "constant" annotation wasn't set. The v6.x expander gets this
wrong, too, for different reasons.

pkgs/racket-test-core/tests/racket/namespac.rktl

@@ -154,6 +154,10 @@
 (test (module-path-index-join ''#%unsafe #f)
       variable-reference->module-path-index (#%variable-reference $$unsafe-fx+))
 
+(test #t variable-reference-constant? (#%variable-reference cons))
+(require (only-in ffi/unsafe _bool))
+(test #t variable-reference-constant? (#%variable-reference _bool))
+
 ;; ----------------------------------------
 
 (module phaser scheme/base 

racket/src/expander/boot/kernel.rkt

@@ -85,7 +85,7 @@
                 (lambda (data-box ns phase-shift phase-level self bulk-binding-registry insp)
                   (when (= 0 phase-level)
                     (for ([(sym val) (in-hash ht)])
-                      (namespace-set-variable! ns 0 sym val)))))
+                      (namespace-set-variable! ns 0 sym val #t)))))
    (module-path-index-resolve mpi)))
 
 (define (declare-reexporting-module! name require-names

racket/src/racket/src/startup.inc

@@ -58672,7 +58672,8 @@ static const char *startup_source =
 " ns_123"
 " 0"
 " sym_104"
-" val_84))"
+" val_84"
+" #t))"
 "(values)))))"
 "(values)))))"
 "(if(not #f)"