[clang] Add SPIRV support to HIPAMD toolchain (PR #75357)

[Alex Voicu via cfe-commits]

AlexVlx wrote:

> > > Is generic the best name here? I feel like that's going to be heavily overloaded. I'd much prefer a new architecture that just treats "SPIR-V" as a single architecture. E.g. `--offload-arch=spirv` or something.
> 
> For HIPAMD toolchain, `--offload-arch=generic` and `--offload-arch=spirv` does not make much difference. However, I understand for OpenMP toolchain `--offload-arch=generic` is probably too ambiguous and `--offload-arch=spirv` is better. I can change it to `spirv`.

Perhaps we should consider prefixing it in some way (e.g. `hip-spirv` or `amd-spirv`) that leaves the door open for some special handling (enable a particular set of extensions only for amdgpu targeting SPIRV, try to deal with missing builtins etc.) / flexibility?

https://github.com/llvm/llvm-project/pull/75357