The definition of the _Atomic(T) macro needs to refer to ::std::atomic,
not some other std::atomic relative to the current namespace.
libstdc++-v3/ChangeLog:
PR libstdc++/115807
* include/c_compatibility/stdatomic.h (_Atomic): Ensure it
refers to std::atomic in the global namespace.
* testsuite/29_atomics/headers/stdatomic.h/115807.cc: New test.
(cherry picked from commit 40d234dd6439e8c8cfbf3f375a61906aed35c80d)
c94c8ff5f5