Fix use of reserved identifier __atomic_compare_exchange by renaming

--- dbinc/atomic.h.orig	2018-03-13 09:44:13.197929296 +0100
+++ dbinc/atomic.h	2018-03-13 09:45:09.582831244 +0100
@@ -144,7 +144,7 @@
 #define	atomic_inc(env, p)	__atomic_inc(p)
 #define	atomic_dec(env, p)	__atomic_dec(p)
 #define	atomic_compare_exchange(env, p, o, n)	\
-	__atomic_compare_exchange((p), (o), (n))
+	__db_atomic_compare_exchange((p), (o), (n))
 static inline int __atomic_inc(db_atomic_t *p)
 {
 	int	temp;
@@ -176,7 +176,7 @@
  * http://gcc.gnu.org/onlinedocs/gcc-4.1.0/gcc/Atomic-Builtins.html
  * which configure could be changed to use.
  */
-static inline int __atomic_compare_exchange(
+static inline int __db_atomic_compare_exchange(
 	db_atomic_t *p, atomic_value_t oldval, atomic_value_t newval)
 {
 	atomic_value_t was;