[SYCL] Prefer `_WIN32` over `WIN32` in `#if` expressions (#16267)
The former is defined by the toolchain while the latter is part of SDK,
so using `_WIN32` is preferrable. The issue was exposed downstream due
to recent changes from https://github.com/intel/llvm/pull/16178 and
https://github.com/intel/llvm/pull/16194.
