[PATCH, libstdc++/testsuite, ping2] 29_atomics/atomic/65913.cc: require atomic-builtins rather than specific target

[Jonathan Wakely]

On 29/06/16 13:49 -0700, Mike Stump wrote:
>Please include the libstdc++ list, they don't all read the other list.

And the documentation clearly says (in two places) that all libstdc++
patches must go to the libstdc++ list.