[compiler-rt] [compiler-rt] Factor HSA layer into common helpers (PR #223532)

[Yaxun Liu via llvm-commits]

yxsamliu wrote:

`RegisterHandler()` updates the handler list under the mutex, but `Dispatch()` reads it without the mutex. This is safe for the current early UBSan registration, but a future late registration could race with dispatch. Could we either require registration before RPC starts or protect both accesses?

https://github.com/llvm/llvm-project/pull/223532