The regression was introduced in intel#18839 and the error looks
like:

```
/include/sycl/multi_ptr.hpp:397:5: error: no matching function for call to '__spirv_ocl_prefetch'
  397 |     __spirv_ocl_prefetch(reinterpret_cast<ptr_t>(get_decorated()), NumBytes);
/include/sycl/multi_ptr.hpp:397:5: note: candidate function not viable: no known conversion from 'ptr_t' (aka 'const __global char *') to 'const __global signed char *' for 1st argument
  397 |     __spirv_ocl_prefetch(reinterpret_cast<ptr_t>(get_decorated()), NumBytes);
```

We previously switched to use `__spirv_ocl_prefetch` that is
automatically defined by the compiler instead of having its
forward-declaration in our headers. However, compiler-provided
declaration does not define an overload for plain `char` for some
reason.

Changing that may trigger some unknown and unwanted side effects, so for
the meantime the problem is worked around by using `unsigned char`
overload instead which should be safe (from strict aliasing rules point
of view).

This is a follow-up for intel#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` 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.