Add support for reference counting using C11 atomics

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

GH: #1500

crypto/ec/ec_key.c

diff --git a/crypto/ec/ec_key.c b/crypto/ec/ec_key.c
index f1f0afb..2d10a8c 100644
--- a/crypto/ec/ec_key.c
+++ b/crypto/ec/ec_key.c
@@ -49,7 +49,7 @@
     if (r == NULL)
         return;
 
-    CRYPTO_atomic_add(&r->references, -1, &i, r->lock);
+    CRYPTO_DOWN_REF(&r->references, &i, r->lock);
     REF_PRINT_COUNT("EC_KEY", r);
     if (i > 0)
         return;
@@ -169,7 +169,7 @@
 {
     int i;
 
-    if (CRYPTO_atomic_add(&r->references, 1, &i, r->lock) <= 0)
+    if (CRYPTO_UP_REF(&r->references, &i, r->lock) <= 0)
         return 0;
 
     REF_PRINT_COUNT("EC_KEY", r);