Chez Scheme: allow some extra inlining of system primitives

Cross-library inlining is willing to inline a procedure body that
refers to a system primitive, but wasn't willing to propagate a
system primitive directly. Enable that, and use it to simplify
`unsafe-struct` inlining.

Related to #3546

racket/src/ChezScheme/s/cp0.ss

@@ -1979,7 +1979,9 @@
             [,pr
               (context-case ctxt
                 [(value tail)
-                 (if (all-set? (prim-mask (or primitive proc)) (primref-flags pr))
+                 ;; formerly constrained to functions marked as `primitive`,
+                 ;; which does not include `system` procedures:
+                 (if (all-set? (prim-mask proc) (primref-flags pr))
                      rhs
                      (residualize-ref maybe-src id sc))]
                 [(test)

racket/src/cs/rumble/struct.ss

@@ -1079,8 +1079,8 @@
 (define (unsafe-struct? v r)
   (#3%record? v r))
 
-(define (unsafe-struct r . args)
-  (#%apply #%$record r args))
+;; internal use only, so doesn't need to have 'unsafe-struct as it's name, etc.:
+(define unsafe-struct #%$record)
 
 (define (unsafe-struct-ref s i)
   (if (impersonator? s)