llgs: modify to accept --native-regs flag.

This is the last flag sent by lldb-platform that was not accepted
by llgs and is accepted by debugserver.

Conditionalized out a bit more code in prep for Windows
support one day based on _WIN32 define.

Updated the lldb-gdbserver usage string to represent
recent updates to command line arguments (and some older
ones like --attach pid).

Implements https://github.com/tfiala/lldb/issues/37