[llvm] TableGen: Handle setting runtime libcall calling conventions (PR #144980)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jul 8, 8:07 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/144980).


https://github.com/llvm/llvm-project/pull/144980