[llvm] [OFFLOAD] Add plugin with support for Intel oneAPI Level Zero (PR #158900)

[Ross Brunton via llvm-commits]

RossBrunton wrote:

Just tried building this - Looks like the platform is returning `OL_PLATFORM_BACKEND_UNKNOWN`, which is hitting the check in initPlugins and causing it to be skipped.

https://github.com/llvm/llvm-project/pull/158900