Properly enable Clang-style atomics when available.
Properly enable Clang-style atomics when available.

In addition to testing against cxx_atomic, we must check c_atomic. The
former is only set when building C++ code. Also use __has_extension
instead of __has_feature. This allows us to use the atomics outside of
C11.

Reported by:	Ariane van der Steldt <ariane stack nl>
PR:		threads/170073

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase