Add the following intrinsics to core::arch::x86_64
:
/// (Module documentation)
///
/// Semantics of volatile memory operations: volatile memory operations (reads and writes)
/// must be emitted by the compiler and cannot be re-ordered across other volatile operations.
///
/// The result of a volatile read is frozen (`freeze` is applied to it). That is,
/// reading uninitialized memory via a volatile read never returns uninitialized memory.