Add amdgpu intrinsics

[Flakebi]

Add intrinsics for the amdgpu architecture.

I’m not sure how to add/run CI (ci/run.sh fails for me e.g. for
nvptx because core cannot be found), but I checked that it compiles
without warnings with

$ CARGO_BUILD_RUSTFLAGS=-Ctarget-cpu=gfx900 cargo check --target amdgcn-amd-amdhsa -Zbuild-std=core

Tracking issue: rust-lang/rust#149988

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[sayantn]

I think it can be "tested" the same way we do core::arch::nvptx (this is also a similar NO_RUN, NO_STD target). For those targets, we just test in CI that it builds. You can just look at how we handle nvptx64-nvidia-cuda in .github/workflows/main.yml. Also you would need to add this target to ci/dox.sh to check that the documentation builds

[Flakebi]

Thanks, I tried to replicate what’s there for nvptx + adding -Ctarget-cpu (as amdgpu doesn’t compile without, it has no generic cpu) + build-std=core.
I still can’t build ci/run-docker.sh or ci/run.sh locally (ci/dox.sh seems to work if I remove s390x), but I think I got it working now.
Sorry for the many force-pushes!

The diff to my first push from when opening this PR is here: https://github.com/rust-lang/stdarch/compare/b3f5bdae0efbdd5f7297d0225623bd31c7fe895b..e1028110e77561574bfb7ea349154d46b5ea7b86