Of course, that's the nature of bootstrapping! If someone managed to erase all the software from all the computers in the world, we'd have to go back and find an "Old world" Mac and use the on-die Forth compiler to write a C compiler so we could start compiling things again.
Otherwise, I'm guessing that for Go 1.6, they'll rely on you having a binary distribution of 1.5 (probably from your distribution, or their website), etc.
That's not true at all, (1) you write the backend for the target machine, (2) you recompile the compiler with the new backend included on a supported host, (3) then use the new compiler to compile itself for the target using the new backend, (4) you now have a compiler that runs on the target.
Same process is used to port C compilers (or any self-hosting compiler, really).