Release our own LLVM build

[xermicus]

Tasks

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1. 1. Fork and port the era compiler LLVM builder
2. 2. Change revive to use those builds. The entire utils directory should be obselete.
3. 3. Remove the vendored compiler-rt and use the one from the revive-llvm CI.
4. 4. Create a test workflow for the revive-llvm builder for Linux, MacOS and Windows and also the builtins.
5. 5. Create a release workflow for the revive-llvm tool itself.
6. 6. Create a release workflow for all supported targets and the builtins. builtins should be built with the MUSL host (see the test in the revive-builder repository).

[xermicus]

Update: I am working on the llvm-builder. I have it in a working state and am currently integrating it as a crate into this repository. This completes steps 1 - 3.

This will fully unblock the release worflows.