lib: use stdatomic.h for memory barriers

C11 gives us APIs for fencing logic.  Use that instead of a
a long, per-architecture, hand curated list of definitions.

lib/sp-callgraph-view.c

@@ -41,8 +41,6 @@
 #include "sp-callgraph-view.h"
 #include "sp-cell-renderer-percent.h"
 
-#include "util.h"
-
 typedef struct
 {
   SpCallgraphProfile  *profile;