Fix stdatomic.h header check for ICC compiler: the ICC implementation
lacks atomic_uintptr_t type which is needed by Python.

Test:

* atomic_int and atomic_uintptr_t types
* atomic_load_explicit() and atomic_store_explicit()
* memory_order_relaxed and memory_order_seq_cst constants

But don't test ATOMIC_VAR_INIT(): it's not used in Python.