Break expectations. Bootstrap it through an esoteric-enough system. Write an Uxn emulator in assembly targeting the cushy environment that UEFI has and you've got a system with graphics, a text editor, a spreadsheet editor, an assembler, games, and maybe even more. I have a Z80-powered email appliance that can be loaded with programs from a connected device. Whoever is breaking my trust in trust surely won't have planned for that.

You can construct a CPU out of an EEPROM, a clock, and a few latches. Connect it to an immediate mode display with a serial interface that doesn't care about being clocked slowly, connect up a buzzer or some blinkenlights for output when you're exceptionally paranoid and can't trust the display controller, make a basic keyboard with a rubber sheet, some wire, and some glue, poke a keyboard driver and a line editor into memory with your DIP switches, crank the clock up to kilohertz (so the keyboard latency is tolerable), and you too can bootstrap a cross-compiler! (Though be aware that the radio interference will be enough for a committed attacker to figure out what you're computing, unless you take measures against that.)

But they're not going to backdoor an Apple ][e, or a random 80m¢ microcontroller, for basically any value of "they"; so you can just use one of those instead, and save yourself the hassle.

Write a subleq interpreter with a magnet and a steady hand? (Hopefully the magnet is not compromised)

By doing it.

Individual cpu instructions, even of a crude old 8-bit cpu with no embedded minix os like today, are both simple enough for a human to manually understand what they do, and useful enough to build crude versions of useful things like an editor, interpreter, or compiler.

You can write a forth-like language or even a c-like language starting from individual cpu instructions that a human can read, understand, and write totally manually, and then use that to build up rapidly all the way to a full modern desktop.

If you were really paranoid about the very act of the initial typing-in, there are any number of ways to store data in a totally brainless eprom or record it to tape or something, and examine it with nothing but some leds, no cpu at all, to verify the bytes are the bytes you want. And you only need to do that for a pretty small number of initial bytes. After that it's all just regular source code which could be written on paper.

Bootstrapping is only an inconvenience problem, not a real problem.

It's not convenient for most people to assemble some bytes into some storage medium and then verify them without simply using a normal untrust-able computer to do it. But it's no problem really if you had some reason to be that careful.

We have tons and tons of backups of clean Linux isos, compilers, etc. The idea that we are going to lose the ability to easily have an uncompromised system is a fairy tale told by the people pushing bootstrapable builds.