[Bug libstdc++/64658] std::atomic_init() undefined

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64658

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Testcase for 4.9 (which doesn't have the fix for PR64940 that allows
std::atomic_int t o be used interchangeably with std::atomic<int>):

#include <atomic>

int main()
{
  std::atomic<int> a;
  atomic_init(&a, 0);
}