[llvm] Reland #118503: [Offload] Introduce offload-tblgen and initial new API implementation (PR #118614)

[Callum Fare via llvm-commits]

callumfare wrote:

@nikic Do you see the same problem with building libomptarget (also part of the offload runtime)? I've had a look and it seems to also pull in the host-built `libLLVMSupport.a` etc. Either way I can see the issue but I'm wondering if there's an existing workaround that I'm not aware of.

I don't think the cross-compilation case is broken because `add_tablegen` respects `LLVM_USE_HOST_TOOLS` although I haven't verified that.

https://github.com/llvm/llvm-project/pull/118614