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Summary: The GPU runs these tests using the files built from the `libc` project. These will be placed in `include/<triple>` and `lib/<triple>`. We use the `amdhsa-loader` and `nvptx-loader` tools, which are also provided by `libc`. These launch a kernel called `_start` which calls `main` so we can pretend like GPU programs are normal terminal applications. We force serial exeuction here, because `llvm-lit` runs way too many processes in parallel, which has a bad habit of making the GPU drivers hang or run out of resources. This allows the compilation to be run in parallel while the jobs themselves are serialized via a file lock. In the future this can likely be refined to accept user specified architectures, or better handle including the root directory by exposing that instead of just `include/<triple>/c++/v1/`. This currently fails ~1% of the tests on AMDGPU and ~3% of the tests on NVPTX. This will hopefully be reduced further, and later patches can XFAIL a lot of them once it's down to a reasonable number. Future support will likely want to allow passing in a custom architecture instead of simply relying on `-mcpu=native`.