[libclc] [libclc] Use generic spirv*-unknown-unknown clang triple for SPIR-V targets (PR #199618)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> The input is generic OpenCL SPIR-V. There is nothing mesa specific about it, but I think it's not a good idea to imply otherwise.

This is mostly just questioning how things are build in mesa. Because if you ever wanted to have custom toolchain handling for building mesa-compatible objects you would want a dedicated triple for it (e.g. `x86_64-unknown-linux-gnu` vs. `x86_64-unknown-linux-musl`). But I would assume that you are hard-coding tool invocations in mesa rather than running it through clang (I would guess because clang cannot consume SPIR-V yet).

I know the mesa component completely changes the ABI in the `amdgcn` step, so mostly this was an assumption that you'd go from mesa SPIR-V to mesa AMDGCN or similar when compiling.

Realistically this PR is all documentation though, I think it's just up to the user as the unknown variant still worked.

https://github.com/llvm/llvm-project/pull/199618