[Rolpa]

But the idea of an OS in and of itself is the ultimate IDE, is it not? I can't say I like the idea of writing software without any system calls at my disposal.

[pjmlp]

That is the case of C, because UNIX is the runtime, hence what was left out of ISO ended up on POSIX.

Any programming language with a richer runtime can be OS agnostic to a certain point. The runtime is the OS.