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/evp/p_lib.c

diff --git a/crypto/evp/p_lib.c b/crypto/evp/p_lib.c
index 9828620..234d05f 100644
--- a/crypto/evp/p_lib.c
+++ b/crypto/evp/p_lib.c
@@ -160,7 +160,7 @@
 {
     int i;
 
-    if (CRYPTO_atomic_add(&pkey->references, 1, &i, pkey->lock) <= 0)
+    if (CRYPTO_UP_REF(&pkey->references, &i, pkey->lock) <= 0)
         return 0;
 
     REF_PRINT_COUNT("EVP_PKEY", pkey);
@@ -392,7 +392,7 @@
     if (x == NULL)
         return;
 
-    CRYPTO_atomic_add(&x->references, -1, &i, x->lock);
+    CRYPTO_DOWN_REF(&x->references, &i, x->lock);
     REF_PRINT_COUNT("EVP_PKEY", x);
     if (i > 0)
         return;