Auto merge of rust-lang#32647 - Amanieu:checked_atomic_intrinsics, r=…

…eddyb

Only allow using the atomic intrinsics on integer types

Using these with non-integer types results in LLVM asserts. Atomic operations on non-integer types will require values be transmuted into an integer type of suitable size.

This doesn't affect the standard library since `AtomicBool` and `AtomicPtr` currently use `usize` for atomic operations.

r? @eddyb

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