Comment by MobiusHorizons

Compiling the code is not the issue. The hard part is the system integration. Most notably the boot process and peripherals. It's not actually hard to compile code for any given ARM or x86 target. Even much less open ecosystems like IBM mainframes have free and open source compilers (eg GCC). The ISA is just how computation happens. But you have to boot the system, and get data in and out for the system to be actually useful, and pretty much all of that contains vendor specific quirks. Its really only the x86 world where that got so standardized across manufacturers, and that was mostly because people were initially trying to make compatible clones of the IBM PC.