Implement atomic_compare_exchange_strong in the atomic compat headers

This fixes building with MSVC (and older GCC versions) after 3e7886db.

include/compat/gcc/stdatomic.h

@@ -44,6 +44,7 @@ typedef unsigned int atomic_uint;
 #define atomic_fetch_sub(p_a, dec)    __atomic_fetch_sub(p_a, dec, __ATOMIC_SEQ_CST)
 #define atomic_exchange(p_a, v)       __atomic_exchange_n(p_a, v, __ATOMIC_SEQ_CST)
 #define atomic_fetch_or(p_a, v)       __atomic_fetch_or(p_a, v, __ATOMIC_SEQ_CST)
+#define atomic_compare_exchange_strong(p_a, expected, desired) __atomic_compare_exchange_n(p_a, expected, desired, 0, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST)
 
 #endif /* !defined(__cplusplus) */