[clang] [llvm] [AMDGPU] Introduce a new generic target `gfx9-4-generic` (PR #115190)

[Shilei Tian via cfe-commits]

shiltian wrote:

### Merge activity

* **Nov 12, 11:06 PM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/115190).


https://github.com/llvm/llvm-project/pull/115190