atomic: fix CAS with armcc.

On the current code, armcc will fail with:
"libavutil/atomic_gcc.h", line 52: Error:  #2771: first argument must be
a pointer to integer or enumeration type

libavutil/atomic_gcc.h

@@ -21,6 +21,8 @@
 #ifndef AVUTIL_ATOMIC_GCC_H
 #define AVUTIL_ATOMIC_GCC_H
 
+#include <stdint.h>
+
 #include "atomic.h"
 
 #define avpriv_atomic_int_get atomic_int_get_gcc
@@ -47,7 +49,13 @@ static inline int atomic_int_add_and_fetch_gcc(volatile int *ptr, int inc)
 static inline void *atomic_ptr_cas_gcc(void * volatile *ptr,
                                        void *oldval, void *newval)
 {
+#ifdef __ARMCC_VERSION
+    // armcc will throw an error if ptr is not an integer type
+    volatile uintptr_t *tmp = (volatile uintptr_t*)ptr;
+    return (void*)__sync_val_compare_and_swap(tmp, oldval, newval);
+#else
     return __sync_val_compare_and_swap(ptr, oldval, newval);
+#endif
 }
 
 #endif /* AVUTIL_ATOMIC_GCC_H */