Rollup merge of #129116 - Zalathar:compiler-rt, r=Mark-Simulacrum

Include a copy of `compiler-rt` source in the `download-ci-llvm` tarball

This will make it possible to experiment with allowing `download-ci-llvm` builds to build `library/profiler_builtins`, without needing to check out the `src/llvm-project` submodule.

By itself, this PR just adds the files to the tarball, but doesn't actually do anything with them. The idea is that once this is merged, it will then be much easier to proceed with work on the necessary bootstrap changes (using the real downloaded tarball), without having to rig up weird hacks to simulate downloading a modified tarball.

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Adding these files to the compressed tarballs appears to increase its size by a negligible amount (<1 MB out of 400/800+ MB).

The uncompressed size is about 14 MB (out of 2+ GB for the whole tarball).

(The excluded test files would have been another 35 MB.)

Author: tgross35