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

[Karol Herbst via cfe-commits]

karolherbst wrote:

> > Could maybe encode the SPIRV-LLVM-Translator vs LLVM backend use by calling it `-llvm` or `-khronos` as part of the triple? And then maybe also encode `opencl` vs `vulkan`, so a triple could look like `spirv64-opencl-khronos` (compiling OpenCL SPIR-V with the SPIRV-LLVM-Translator) or `spirv64-vulkan-llvm` (compiling Vulkan SPIR-V with the llvm SPIR-V backend)?
> 
> `spirv64-opencl-khronos` is not a valid triple as `opencl` isn't vendor and `khronos` isn't os. Vulkan variant triple is `spirv64-unknown-vulkan`.

oh right, I meant the other way around.

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