Don't advertise :alien-callbacks on mips

crossbuild-runner/backends/mips/features

@@ -1,2 +1 @@
 :gencgc :use-cons-region :soft-card-marks
-:alien-callbacks

src/compiler/mips/c-call.lisp

@@ -322,6 +322,7 @@
         `(deref (sap-alien (sap+ ,sap ,(+ offset delta)) (* ,type)))))
      delta)))
 
+;;; FIXME: doesn't actually work (see tests)
 ;;; Returns a vector in static space containing machine code for the
 ;;; callback wrapper
 #-sb-xc-host

tests/foreign-stack-alignment.impure.lisp

@@ -68,8 +68,7 @@
   (assert (= *good-offset* (stack-alignment-offset *required-alignment*))))
 
 #+alien-callbacks
-(with-test (:name :callback
-            :broken-on :mips)
+(with-test (:name :callback)
   (assert (= *good-offset*
              (with-alien-callable ((callback int ()
                                      (stack-alignment-offset *required-alignment*)))