Allow use of gcc-style __atomic* builtins in older clang versions.

Avoid use of const pointers with atomic_load because strictly C11
compliant compilers error on this. Using const became allowed in C17:
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2244.htm#dr_459
--- lib/isc/include/isc/stdatomic.h.orig	2022-10-10 18:01:57.000000000 +1100
+++ lib/isc/include/isc/stdatomic.h	2022-11-03 20:54:26.000000000 +1100
@@ -21,7 +21,9 @@
 #endif /* HAVE_UCHAR_H */
 
 /* GCC 4.7.0 introduced __atomic builtins, but not the __GNUC_ATOMICS define */
-#if !defined(__GNUC_ATOMICS) && __GNUC__ == 4 && __GNUC_MINOR__ >= 7
+/* Clang 3.1 and later have the builtins but none define __GNUC_ATOMICS */
+#if !defined(__GNUC_ATOMICS) && ((__GNUC__ == 4 && __GNUC_MINOR__ >= 7) \
+    || defined(__clang__))
 #define __GNUC_ATOMICS
 #endif
 
--- lib/isc/netmgr/netmgr-int.h.orig	2022-10-10 18:44:05.000000000 +1100
+++ lib/isc/netmgr/netmgr-int.h	2022-11-03 20:57:03.000000000 +1100
@@ -244,7 +244,7 @@ typedef struct isc__networker {
 #define NMHANDLE_MAGIC ISC_MAGIC('N', 'M', 'H', 'D')
 #define VALID_NMHANDLE(t)                      \
 	(ISC_MAGIC_VALID(t, NMHANDLE_MAGIC) && \
-	 atomic_load(&(t)->references) > 0)
+	 atomic_load((isc_refcount_t *)&(t)->references) > 0)
 
 typedef void (*isc__nm_closecb)(isc_nmhandle_t *);
 typedef struct isc_nm_http_session isc_nm_http_session_t;