Currently 64-bit atomic<T> on x86 uses _InterlockedCompareExchange64 for any store operation, even for memory_order_relaxed.

Whereas for load it uses __iso_volatile_load64 intrinsic.

__iso_volatile_store64 appears to be working on x86. Though it was blamed to cause ICE in revision bb746ac.

Clarify the status of __iso_volatile_store64 on x86 and use it if it is possible.