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How to Install Linux From Scratch - tony
(youtube.com)
A community for everything relating to the GNU/Linux operating system (except the memes!)
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Not far if you dig out the last C compiler written in assembly by hand. Use that to bootstrap GCC, possibly.
Not sure if something like that would even support x86 or C99, so would probably still need an older mainframe and early GCC source tree. Could probably get by with a virtual emulator for the former.