I imagined going a slightly different route.

A minimal Forth can be written in assembly and in itself. It suffices to write a console using a serial port, a primitive FAT filesystem to access SPI Flash, and maybe even an interface to USB mass storage.

Forth is not very easy to audit, but likely still easier than raw assembly.

One can write a C compiler right on top of that, sufficient to compile TCC.

Alternatively, a simple Lisp can be written on top of the Forth, it's much simpler than writing it in assembly. Using the Lisp, a much more understandable and auditable C compiler can be written.

Much of the Forth, all of the Lisp, and much of the C compiler (except code generation) would be portable and reusable across multiple architectures, without the need to audit them fully every time.

The fun part here is (potentially) not using QEMU and cross-compilers, and running everything on a sufficiently powerful target hardware, for the extra paranoid.