Not at all, cross platform means executing the same application in many platforms, regardless of the hardware and OS specific features of each platform.

Cross-compilation is useless if you don't actually get to executed the created binaries in the target platform.

Now, how do you intend to compile from GNU/Linux into z/OS, so that we can execute the generated binary out from the C compiler ingesting the code written in GNU/Linux platform, in the z/OS language environment inside an enclave, not configured in POSIX mode?

Using z/OS, if you're feeling more modern, it can be UWP sandboxed application with identity in Windows.

> cross platform means executing the same application in many platforms, regardless of the hardware and OS specific features of each platform.

That is a better definition yes. But it's still not synonymous with cross-compilation, obviously. Most cross-platform apps are not cross-compiled because it's usually such a pain.