[SYCL] fix for __sycl_unregister_lib() on Windows and tests

[cperkinsintel]

__sycl_unregister_lib() is not being called on Windows when using shared libraries due to a limitation in LLVM/clang To work around this, on Windows we register both __sycl_register_lib() and an std::atexit handler that will call __sycl_unregister_lib() . Further, it was discovered that freeing of the device globals during device images destruction is duplicate and unnecessary. The ~context_impl destructor handles that (and handles it correctly, because a context is needed to free USM memory). So we remove the unneeded duplication. Adding a test that stresses __sycl_unregister_lib() and makes sure there are no resource leaks

[cperkinsintel]

PVC failures is unrelated: #19662

[steffenlarsen]

Yes! Looks like a solid solution to me! 🚀

[cperkinsintel]

@intel/dpcpp-tools-reviewers - ping for review

[sarnex]

im not sure i understand this comment, to me it seems like the abi would be different so we can't use i32 (ptr) to represent void(*)()

[cperkinsintel]

I simplified it. I just wanted to describe atexit.