Fix tst-obprintf - and mcheck in general

[Roland McGrath]

> On 05/03/2012 08:41 PM, Roland McGrath wrote:
> >> +#define malloc_opt_barrier(x) \
> >> +({ __typeof (x) __x = x; __asm ("" : "+m" (__x)); __x; })
> >
> > This needs a comment.
> 
> Like this?

Good enough.