add gpu device side instructions

[ZuseZ4]

WIP, I will develop this along with rust-lang/rust#142696

command:

RUSTFLAGS="-Ctarget-cpu=gfx90a --emit=llvm-ir" cargo +offload build -Zunstable-options -r -v --target amdgcn-amd-amdhsa -Zbuild-std=core

[tshepang]

feel free to have these changes on the other side (in the guide subtree, together with the linked pr), to avoid 2 separate reviews

[ZuseZ4]

Good to know @tshepang, thanks! In the past I just merged my own PRs here, but if our infra can handle it, I might start doing them together, so my reviewers on the rustc side have an easier time understanding my gpu work.

[ZuseZ4]

I think these instructions are much to involved for now for any user to follow along, but it's still valuable, either to talk about them with other gpu compiler people, or at the least for me to not forget how to use that code.

Over the next months I will work on reproducing more of the clang infrastructure in rustc, this way we can gradually simplify the instructions here.

[rustbot]

Thanks for the PR. If you have write access, feel free to merge this PR if it does not need reviews. You can request a review using r? rustc-dev-guide or r? <username>.

[rustbot]

⚠️ Warning ⚠️

• This PR is based on an upstream commit that is older than 7 days.

  It's recommended to update your branch according to the rustc-dev-guide.