[SYCL 2020] Add atomic_ref, refactor atomics, implement atomic builtins for all types and operations

[illuhad]

• Add SYCL 2020 atomic_ref
• Refactor atomics in a fashion similar to how math builtins work: A common atomic_builtins.hpp header includes backend-specific headers which provide __hipsycl_atomic_* backend builtins. These can then be used by both atomic and atomic_ref classes.
• Add support for previously unsupported atomic operations, such as load/store on GPU, or floating point atomics on CPU by adding appropriate CAS emulations. With this PR, all atomic operations are supported for all types on all backends (except SPIR-V, which requires more work in a separate PR)
• Add test coverage

[illuhad]

@hipSYCL-ci-bot test again.