The requirements for BIND 9.17+ now requires C11 support from the
compiler, so we can safely drop most of the stdatomic.h shims from
lib/isc/unix/include/stdatomic.h.
This commit removes support for clang atomic builtins (clang >= 3.6.0
includes stdatomic.h header) and for Gcc __sync builtins.
The only compatibility shim that remains is support for __atomic
builtins for Gcc >= 4.7.0 since CentOS 7 still includes only Gcc 4.8.1
and the proper stdatomic.h header was only introduced in Gcc >= 4.9.
54c389dbc0