[lldb] Skip TestIOHandlerPythonREPLSigint if *host* is linux

The current dectorator (@skipIfLinux) will skip the test if the lldb
platform is the linux platform, but the issue is with the OS that lldb
is running on, not the OS that lldb is debugging. Update the decorator
to skip the test if the host is Linux.

Thank you Ted Woodward for pointing this out.