Mock build on non-Linux platforms

[adamierymenko]

When editing or working on a non-Linux platform like macOS, it's annoying to have the build errors happen. It would be better if Glommio just mocked out everything on non-Linux platforms with code that panicked on execution and disabled parts of the build that fail on other platforms. It would build but would not run, panicking with "glommio requires Linux and io_uring" or something.