[llvm] [Offload] Use `amd_signal_async_handler` for host function calls (PR #154131)

[Ross Brunton via llvm-commits]

RossBrunton wrote:

So it turns out that callbacks used by the AMD RTL actually do run in response to the jobs. The docs I was looking at didn't include `amd_signal_async_handler` so I wasn't aware such a thing existed.

https://github.com/llvm/llvm-project/pull/154131