Return an actual value instead of whatever set! returns (#63)

Don't use `set!` as the last statement, but instead return (some arbitrary) value.

src/compiler.scm

@@ -6985,7 +6985,8 @@
 (define asm-line '())
 (define (asm-open-output-file filename)
   (set! asm-output '())
-  (set! asm-line '()))
+  (set! asm-line '())
+  filename)
 (define (asm-close-output-port asm-port) #f)
 (define (asm-newline asm-port) (asm-display char-newline asm-port))
 (define (asm-display obj asm-port)