[llvm-branch-commits] [libc] Readd instructions on building kernel headers from sources (PR #193657)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/193657

These existed in this page before
aa0e429576cfb496e66f6bbe76df8982aab6b083. Readd them given I enjoyed
being able to copy and paste them and they're short/simple enough that I
don't think there's a good reason not to just include them.