Display local kernel version only when non-remote in PlatformLinux.

Summary:
This is part of the Linux remote platform work. Displaying the local
kernel information when remote debugging doesn't make sense, so we
should verify if we are in host mode before doing so.

Test Plan:
Connect to a remote linux platform mode daemon with `platform select
remote-linux` followed by `platform connect ...`, and look at the output
of `platform status`.

Reviewers: tfiala, clayborg, vharron, compnerd

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D5928