AtomicAccess::xchg is only required to support 4 bytes and sizeof(intptr_t) size.
This restriction added a lot of extra logic to the Atomic implementation because
we have a set of features we must support (including compare exchange) for 1, 4 and 8 byte atomics on all platforms. We have some checks for unsupported 8 byte compare exchange (`VM_Version::supports_cx8()`), but the Atomic class does not try to handle these for generating its supported functions. On such a platform we would more than likely get a linking error.

I propose we change requirement for exchange to 1, 4 and 8 bytes to achieve parity with compare exchange. Initially by implementing exchange via the `AtomicAccess::XchgUsingCmpxch`. And have follow up RFEs for each applicable platform where we specialize `AtomicAccess::PlatformXchg<1>`.

This enhancement both simplifies the Atomic implementation and provides exchange capabilities for types like `bool` and enums represented by a byte.