Bootstrapping and language safety are orthogonal. C is unsafe and still you can't bootstrap it if you don't already have a compiler which can compile your C compiler.
According to that logic even assembly is not low level enough because you need an assembler to make a runnable program out of it.
Source code available,
How do you bootstrap languages like Rust? Another 'safe' language? What about that one?
Someone somewhere has to be working at the asm level.