Use atomic_element_unordered_copy_memory_nonoverlapping to read the data

[htfy96]

rust-lang/rust#58599
https://internals.rust-lang.org/t/expose-llvm-atomic-memcpy-in-intrinsics/9466/4

The volatile_read here is an questionable operation which depends on internal implementations to synchronize reads with the previous read-acquire on seqlock.

Ref: #2

[Amanieu]

AFAIK these intrinsics don't work correctly at the moment or generate poor code quality. But it's true that eventually we will want to use them.