[clang] [libc] [llvm] [libc] Add basic support for building SPIR-V libraries (PR #181049)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> When adding a new target our policy says we need CI and at least one maintainer (see: https://libc.llvm.org/porting.html). Are we counting SPIR-V as part of GPU under @jhuber6 ?

Yeah, it's fundamentally a GPU architecture. We will 100% need CI, but I'm hoping that being able to build the libraries will make it easier to test. I trust that some people on the SPIR-V team like @sarnex or @fineg74 will do the necessary work to get the `llvm-gpu-loader` to work here. It will be a lot simpler once I land https://github.com/llvm/llvm-project/pull/162739

https://github.com/llvm/llvm-project/pull/181049