[SYCL] Initial support for sycl_ext_oneapi_atomic16

[KornevNikita]

Spec: https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/proposed/sycl_ext_oneapi_atomic16.asciidoc

This patch adds support for atomic_ref half specialization and introduces a new atomic16 aspect. There is no yet native support for 16bit float compare_exchange function, extension is needed.

16bit int (incl. bfloat16) atomics will be supported later, when there is a spec allowing these instructions.