[New offload driver][llvm-link][SYCL] Use LTO instead of llvm-link to link device input bitcodes

[asudarsa]

When using new offload model for SYCL offload, clang-linker-wrapper gathers all bitcode files and uses llvm-link to link them together. This PR has the following changes:

1. Add SPIRV target as experimental target to the SYCL compiler build
2. Use SPIR-V backend LTO to link all device input bitcodes instead of llvm-link. LTO can be hooked up to exit early without calling device backend

Thanks

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This pull request was closed because it has been stalled for 30 days with no activity.