What's so hard about it? Compiler construction is well researched topic and taught in the universities. I made toy language compiler as a student. May be I'm underestimating this task, but I think that I can build some simple C compiler which will output trivial assembly. Given my salary of $2500, that would probably take me around a year, so that's pretty close LoL.
Everybody talks as Linux is the most difficult thing to compile in the world. The reality is that linux is well written and designed with portability with crappy compilers in mind from the beginning.
Also, the booting part, as stated some times, is discutable.
The reality is you can build Linux with gcc and clang. And that’s it. Years ago you could use Intel’s icc compiler, but that stopped being supported. Let’s stop pretending it’s an undergrad project.