[llvm] [SYCL] Add libsycl, a SYCL RT library implementation project (PR #144372)

[Arvind Sudarsanam via llvm-commits]

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asudarsa wrote:

This doc can be reformatted as follows:

Main Heading: SYCL runtime implementation
Sub-topics:
1 .Current Status
2. Overview
3. Build steps
Thanks

https://github.com/llvm/llvm-project/pull/144372