cs: fix allocation mode for _string/utf-16

Closes #3153

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

@@ -745,6 +745,9 @@
   ;; Check a corner of UTF-16 conversion:
   (test "\U171D3" cast (cast "\U171D3" _string/utf-16 _gcpointer) _gcpointer _string/utf-16))
 
+;; strings can be cast
+(test "heλλo" cast (cast "he\u3bb\u3bbo" _string/utf-16 _gcpointer) _gcpointer _string/utf-16)
+
 ;; check async:
 (when test-async?
   (define (check async like)

racket/src/cs/rumble/foreign.ss

@@ -674,7 +674,10 @@
 
 (define (ctype-malloc-mode c)
   (let ([t (ctype-our-rep c)])
-    (if (or (eq? t 'gcpointer) (eq? t 'scheme))
+    (if (or (eq? t 'gcpointer)
+            (eq? t 'scheme)
+            (eq? t 'string/ucs-4)
+            (eq? t 'string/utf-16))
         'nonatomic
         'atomic)))