[SYCL] Implement sycl_khr_queue_empty_query

[HPS-1]

Implement khr_empty() under sycl_khr_queue_empty_query (see KhronosGroup/SYCL-Docs#700), which is essentially the same as ext_oneapi_empty() under sycl_ext_oneapi_queue_empty (https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/supported/sycl_ext_oneapi_queue_empty.asciidoc), and create respective test cases.

Note: the change of adding __DPCPP_ENABLE_UNFINISHED_KHR_EXTENSIONS in sycl/source/CMakeLists.txt is to make sure khr_empty's implementation get successfully compiled when compiling runtime. (Otherwise there will be a "no declaration matches" error since the declaration for khr_empty in sycl/include/sycl/queue.hpp is guarded under #ifdef __DPCPP_ENABLE_UNFINISHED_KHR_EXTENSIONS)

[againull]

@HPS-1 overall looks good, but should we wait for khronos specification to be merged first?

[HPS-1]

@HPS-1 overall looks good, but should we wait for khronos specification to be merged first?

As suggested by Alexey, there's actually a 'deadlock' here, because the SYCL-Docs PR won't be merged until we implement that. So as described in https://github.com/intel/llvm/blob/sycl/sycl/doc/developer/KHRExtensions.md, we need to put our implementation under macro __DPCPP_ENABLE_UNFINISHED_KHR_EXTENSIONS and then proceed with the merge. I've added the needed macros in this PR.