Update the remote debugging guide to include methods to specify a gdb-remote port

[xusheng6]

While running lldb-server in server mode, we can specify the gdbremote port to use via --gdbserver-port 1338. This is especially useful when the remote host is behind a firewall and we need to port forward the port.

More info: https://lldb.llvm.org/man/lldb-server.html#gdb-server-connections