Minias can assemble itself
...but it's written in C and uses a parser generator? IMHO it feels a bit backwards --- and perhaps even a bit cheating if you're doing this for a "bootstrap pilgrimage" --- to write a lower-level tool in a higher-level language. On the other hand, the same author also links to a C compiler in C, without a parser generator: https://github.com/michaelforney/cproc