nasm is an assembler though, not a ‘languages’

That's like saying "clang is a compiler though, not a language". It's correct but completely beside the point. Unless you're writing a compiler, "cross platform assembler" is kind of an insane thing to ask for. If want to learn low level programming, pick a platform. If you are trying to write a cross-platform program in assembly, WHY!? Unless you're writing a compiler. But even then, in this day and age using a cross-platform assembler is still kind of an insane way to approach that problem; take a lesson from decades of progress and do what LLVM did: use an intermediate representation.