[clang] [llvm] runtimes: Pass CMAKE_SYSTEM_NAME based on target triple (PR #203504)

[Matt Arsenault via cfe-commits]

arsenm wrote:

* **#203504** <a href="https://app.graphite.com/github/pr/llvm/llvm-project/203504?utm_source=stack-comment-icon" target="_blank"><img src="https://static.graphite.dev/graphite-32x32-black.png" alt="Graphite" width="10px" height="10px"/></a> 👈 <a href="https://app.graphite.com/github/pr/llvm/llvm-project/203504?utm_source=stack-comment-view-in-graphite" target="_blank">(View in Graphite)</a>
* `main`
<hr />

## How to use the Graphite Merge Queue

Add the label <kbd>FP Bundles</kbd> to this PR to add it to the merge queue.

You must have a Graphite account in order to use the merge queue. Sign up using [this link](https://app.graphite.com/invite/github/llvm?ref=merge-queue-instructions-in-stack-comment&prId=203504).

<h2></h2>
<sub>

An organization admin has enabled the [Graphite Merge Queue](https://graphite.dev/docs/graphite-merge-queue) in this repository.

</sub>

<sub>

Please do not merge from GitHub as this will restart CI on PRs being processed by the merge queue.

</sub>

<h2></h2>

This stack of pull requests is managed by <a href="https://graphite.dev?utm-source=stack-comment"><b>Graphite</b></a>. Learn more about <a href="https://stacking.dev/?utm_source=stack-comment">stacking</a>.
<!-- Current dependencies on/for this PR: -->

https://github.com/llvm/llvm-project/pull/203504