[llvm] SafeStack: Check if __safestack_pointer_address is available (PR #147917)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jul 15, 2:26 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/147917).


https://github.com/llvm/llvm-project/pull/147917