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/dso/dso_lib.c

diff --git a/crypto/dso/dso_lib.c b/crypto/dso/dso_lib.c
index 52816df..8f185b3 100644
--- a/crypto/dso/dso_lib.c
+++ b/crypto/dso/dso_lib.c
@@ -65,7 +65,7 @@
     if (dso == NULL)
         return (1);
 
-    if (CRYPTO_atomic_add(&dso->references, -1, &i, dso->lock) <= 0)
+    if (CRYPTO_DOWN_REF(&dso->references, &i, dso->lock) <= 0)
         return 0;
 
     REF_PRINT_COUNT("DSO", dso);
@@ -107,7 +107,7 @@
         return 0;
     }
 
-    if (CRYPTO_atomic_add(&dso->references, 1, &i, dso->lock) <= 0)
+    if (CRYPTO_UP_REF(&dso->references, &i, dso->lock) <= 0)
         return 0;
 
     REF_PRINT_COUNT("DSO", r);