[llvm] SPIRV: Define runtime libcalls to be (almost) empty (PR #148631)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jul 15, 9:49 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/148631).


https://github.com/llvm/llvm-project/pull/148631