Add support for reference counting using C11 atomics

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>

GH: #1500

ssl/ssl_cert.c

diff --git a/ssl/ssl_cert.c b/ssl/ssl_cert.c
index 9d35957..f26e876 100644
--- a/ssl/ssl_cert.c
+++ b/ssl/ssl_cert.c
@@ -235,7 +235,7 @@
     if (c == NULL)
         return;
 
-    CRYPTO_atomic_add(&c->references, -1, &i, c->lock);
+    CRYPTO_DOWN_REF(&c->references, &i, c->lock);
     REF_PRINT_COUNT("CERT", c);
     if (i > 0)
         return;