Setup a release action for producing binaries

[spiessimon]

This PR adds a release action that compiles LLVM and attaches the resulting binaries to the release. It takes the branch from which the release is made into account and incorporates it into the name of the attached tarball.

It currently uses the following naming format for the binaries: ${{RELEASE_TAG}}-llvm-${{OS}}-${{ARCH}}-${{BRANCH}}, where RELEASE_TAG is the tag of the release, OS is currently only Linux, ARCH is currently x86 64-bit and aarch64, and BRANCH is the source branch of the release. This turns the resulting URLs into: ocaml-flambda/llvm-project/releases/download/${{ RELEASE_TAG }}/${{BINARY_NAME}}.tar.gz, where BINARY_NAME is the name of the binary described above.

Testing I've created an example release here that used the action to produce the binaries: https://github.com/ocaml-flambda/llvm-project/releases/tag/test-release-v7. I've tried out the resulting binary in the following run: https://github.com/ocaml-flambda/llvm-project/actions/runs/16775365155. (These were tried before the name format change discussed below.)

[xclerc]

I have checked that the installed
packages and build steps are in
line with what I am using locally.

As discussed off-line, we may want
to add the release part of the URL
to the file name, but it is not blocking
(and quite possibly not desirable).

[spiessimon]

Added the tag to the name of the tar ball.