[lldb] Fix bug in skipIfRosetta decorator

Currently, the skipIfRosetta decorator will skip tests with the message
"not on macOS" on all platforms that are not `darwin` or `macosx`.
Instead, it should only check the platform and architecture when running
on these platforms.

This triggers for example when running the test suite on device.

Differential revision: https://reviews.llvm.org/D85388