[SYCL] Fix `sycl::atomic` regression

[AlexeySachkov]

This is a follow-up for intel/llvm#18839 which broke host compilation of sycl::atomic.

The actual fix is just an addition of #include <sycl/__spirv/spirv_ops.hpp> to sycl/atomic.hpp.

However, I didn't want spirv_ops.hpp to appear again in sycl/detail/core.hpp and therefore I went further to make sure that sycl/atomic.hpp isn't used by sycl/accessor.hpp which prompted other changes in this PR.