Enable lldb-platform exe support for Linux.

This change enables lldb-platform for Linux.  In addition, it does the following:

* fixes Host::GetLLDBPath() to work on Linux/*BSD for ePathTypeSupportExecutableDir-relative paths.

* adds more logging and comments around lldb-platform startup and remote lldb-platform usage.

* refactors lldb-platform remote-* support for Darwin and Linux into PlatformPOSIX.  This, in theory, is the bulk of what is needed for *BSD to make remote connections to lldb-platform as well (although I haven't tested that yet).  FreeBSD can make similar changes to their Platform* as was made here for PlatformLinux to pick up the rest of the bits.

* teaches GDBRemoteCommunication to use lldb-gdbserver for non-Apple hosts.