Make GCC atomics the default atomic implementation [5.0.x]

[devreal]

Access to C11 _Atomic variables incurs sequential memory ordering, which compiles to atomic operations and in many cases is not desired (initialization, non-multi-threaded runs). This change makes GCC atomics the default and uses C11 atomics as a fallback.

This is a backport of https://github.com/open-mpi/ompi/pull/10724 to v5.0.x and includes the follow-up fix in https://github.com/open-mpi/ompi/pull/10819 for https://github.com/open-mpi/ompi/issues/10797.

Signed-off-by: Joseph Schuchart [email protected]