[libc] Add instructions for linux headers

[sivachandra]

libc/docs/full_host_build.rst

98–109

Should wording here should imply, "here's one way to do it but it's up to you to figure out the best way for your system." So, something like, One way to install on a Debian-based x86_64 system is as follows:

<the command you have>

It is left to the reader to figure out the best way to install Linux headers on their system.

libc/docs/full_host_build.rst

 toolchain with which you can build practical/real-world C applications. See
 `<https://github.com/llvm/llvm-project/tree/main/libc/examples>`_ for examples
 of how to start using this new toolchain.
 
 Linux Headers
 =============
 
 If you are using the full libc on Linux, then you will also need to install
-Linux headers in your sysroot. It is left to the reader to figure out the best
-way to install Linux headers on the system they want to use the full libc on.
+Linux headers in your sysroot.  The way to do this varies per system.
+
+These instructions should work on a Debian-based x86_64 system:
+
+.. code-block:: sh
+
+   $> apt download linux-libc-dev
+   $> dpkg -x linux-libc-dev*deb .
+   $> mv usr/include/* /path/to/sysroot/include
+   $> rm -rf usr linux-libc-dev*deb
+   $> ln -s x86_64-linux-gnu/asm ~/Programming/sysroot/include/asm
 
 Using your newly built libc
 ===========================
 
 You can now use your newly built libc nearly like you would use any compiler
 invocation:
 
 .. code-block:: sh
 
    $> /path/to/sysroot/bin/clang -static main.c
 
 .. warning::
    Because the libc does not yet support dynamic linking, the -static parameter
    must be added to all clang invocations.