Offload host2

[ZuseZ4]

r? @oli-obk

A follow-up to my previous gpu host PR. With this, I can (in theory) run a sufficiently simple Rust function on GPUs. I tested it on AMD, where the amdgcn tartget of rustc causes issues due to Addressspace castings, which might not be valid. If I (manually) fix them, I can run the generated IR on an AMD GPU. This should conceptually also work on NVIDIA or Intel. I updated the dev-guide acordingly: https://rustc-dev-guide.rust-lang.org/offload/usage.html

I am unhappy with the amount of standalone functions in my offload code, so in my second commit I bundled some of the code around two structs which are Rust versions of the LLVM/Offload structs which they represent. The structs themselves only have doc comments. Since I directly lower everything to llvm-ir I didn't saw a big value in modelling the struct member variables.

[bors]

☔ The latest upstream changes (presumably #143026) made this pull request unmergeable. Please resolve the merge conflicts.

[ZuseZ4]

We'll still need #143684 to properly recognize our GPU hardware and run the binary on end-user hardware, but here I'll only add codegen tests, so it should work fine for CI.

[ZuseZ4]

related (also wip) rustc-dev-guide update: rust-lang/rustc-dev-guide#2524

[ZuseZ4]

It turns out with enough bugfixes on the host code, this is already enough to launch and run some code.
I moved more code into a follow-up pr.
r? @oli-obk

[rustbot]

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[ZuseZ4]

cc @Kobzol CI failure (failed to clean up disk space), seems unrelated to my work.

[Kobzol]

Yeah that's spurious.