I know that assembly language is typically not cross-platform. And even with things like NASM you would still need different code for different architectures and platforms which have different runtimes and implementations of things such as interrupts. But if someone wanted to program in assembly language because they liked it, is there any implementation of a cross-platform cross-architecture assembly language?
Edit:
What about not assembly in the traditional sense, but a low-level programming language which looks a lot like assembly?
I think Donald Knuth's MMIX is what you may be interested in. Knuth writes programs in his The Art of Computer Programming book in this machine/assembly language. To date no CPU supports it directly. There are only emulators. Oh, someone made an FPGA that can run it. But that's about it.