internal/refcount.h: allow non-atomic build

Configure with -DOPENSSL_DEV_NO_ATOMICS and you get refcount without
atomics.  This is intended for internal development only, to check the
refcounting is properly coded.  It should never become a configuration
option, hence the name of the macro.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/8479)