Release our own LLVM build

[xermicus]

### Tasks
- [x] 1. Fork and port the era compiler LLVM builder
- [x] 2. Change revive to use those builds. The entire utils directory should be obselete.
- [x] 3. Remove the vendored compiler-rt and use the one from the revive-llvm CI.
- [ ] 4. Create a test workflow for the revive-llvm builder for Linux, MacOS and Windows: https://github.com/paritytech/revive/pull/164
- [x] 5. ~~Create a release workflow for the revive-llvm tool itself.~~
- [ ] 6. Create a release workflow for LLVM.

Considerations regarding step 6.:

• The tarball for each architecture should contain the target-final directory only.
• emscripten and musl builds are cross compilation artifacts. The build host architecture shouldn't matter (though it should probably be built on a ubuntu host anyways).

[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.

[xermicus]

#154 solves 1 - 3

Should be mergeable sson, just got a really weird solc error on the CI I can't reproduce locally (it is seemingly entirely unrelated to to changes in our build process)

[xermicus]

As discussed with @mutantcornholio. LLVM build artifacts will be released together with the binaries.