[llvm] [libc][bazel] Add a repo with linux kernel UAPI headers (PR #216867)

[Jordan Rupprecht via llvm-commits]

https://github.com/rupprecht commented:

I'm not very familiar w/ setting up repos like this, so I had to look some stuff up.

One thing I'm wondering is what the workflow is going to be, and how that would affect running on any non-linux platform. Currently this has no effect, but I assume you want to have some libc targets depend on it, in which case you need to properly guard things so that libc rules only depend on it in situations where someone is also setting `LINUX_UAPI_INCLUDE_DIR`. Or if you use #216868 to make this not configurable, is that something that would work on all platforms? And does #216868 allow sufficient configurability, e.g. do you ever need to change which version of linux headers you target, or can you arbitrarily pick anything recent enough?

https://github.com/llvm/llvm-project/pull/216867