work around missing RLIMIT_AS

Index: test-suite/standalone/test-stack-overflow
--- test-suite/standalone/test-stack-overflow.orig
+++ test-suite/standalone/test-stack-overflow
@@ -32,10 +32,10 @@ exec guile -q -s "$0" "$@"
 ;; 100 MB.
 (define *limit* (* 100 1024 1024))
 
-(call-with-values (lambda () (getrlimit 'as))
+(call-with-values (lambda () (getrlimit 'stack))
   (lambda (soft hard)
     (unless (and soft (< soft *limit*))
-      (setrlimit 'as (if hard (min *limit* hard) *limit*) hard))))
+      (setrlimit 'stack (if hard (min *limit* hard) *limit*) hard))))
 
 (define (test)
   (catch 'stack-overflow
