[clang] [clang][driver] Use rva22u64_v as the default march for Fuchsia targets (PR #131183)

[Paul Kirth via cfe-commits]

ilovepi wrote:

### Merge activity

* **Mar 17, 7:02 PM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/131183).


https://github.com/llvm/llvm-project/pull/131183