[clang] [llvm] UEFI backend for x86_64 (PR #109320)

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Prabhuk wrote:

> Some of the clang changes seem kinda _odd_, maybe some of this work and #120632 could be conjoined? The LLVM stuff itself looks great, it's mainly the target stuff in clang seems odd to me.

I can drop the Clang changes in this PR once #120632 lands. This is a draft PR.

https://github.com/llvm/llvm-project/pull/109320