WASI: Fix Platform.exit's infinite recursion

[kkebo]

This issue was introduced by #504.

When the target platform is WASI, Platform.exit causes infinite recursion because it calls itself.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test

[natecook1000]

Thanks, @KKK669!