Use N_SYMSIZE instead of directly accessing ‘struct exec’

On Linux alpha ‘struct exec’ does not have an a_syms member, and the
N_SYMSIZE is mapped to 0.

src/nlist.c

@@ -155,7 +155,7 @@ __aout_fdnlist(fd, list)
 	}
 
 	symoff = N_SYMOFF(*exec);
-	symsize = exec->a_syms;
+	symsize = N_SYMSIZE(*exec);
 	stroff = symoff + symsize;
 
 	/* find the string table in our mmapped area */