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

[Jackson Stogel via llvm-commits]

jtstogel wrote:

FYI, pulling headers from a hermetic source like in https://github.com/llvm/llvm-project/pull/216868 will probably be preferable for running CI. I'll send that for review before adding any of libc's full-build tests to our bazel CI. Downloading + building UAPI headers is a bit slow, so this PR allows injecting a local include directory during development.

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