Support atomic types

[joshlf]

Progress

• Clarify atomic bit validity rust-lang/rust#121943
• Trait implementations
  • AtomicBool
    • KnownLayout
    • TryFromBytes / FromZeros
    • IntoBytes
    • Unaligned
  • AtomicU8, AtomicI8
    • KnownLayout
    • TryFromBytes / FromZeros / FromBytes
    • IntoBytes
    • Unaligned
  • AtomicUN / AtomicIN (N > 8)
    • KnownLayout
    • TryFromBytes / FromZeros / FromBytes
    • IntoBytes
  • AtomicPtr
    • KnownLayout
    • TryFromBytes / FromZeros
• Support platforms for which target_has_atomics is spuriously false (see below)

Details

target_has_atomics

On some platforms, cfg(target_has_atomics) is disabled despite atomics support. For example, thumbv6m-none-eabi supports AtomicU32 but cfg(target_has_atomic = "32") is false on that platform. We will be able to support these cases once cfg(target_has_atomic_load_store) is stabilized.