Add ProcessGDBRemote::GetAuxvData() and fix multiple-packet concatenation for binary data.

ProcessGDBRemote::GetAuxvData obtains the auxv from a remote gdbserver (via a binary-data packet), and returns the data as a DataBufferSP.

The patch includes a small fix to GDBRemoteCommunicationClient::SendPacketsAndConcatenateResponses() to support binary file format packet returns (by not assuming each binary packet is a null-terminated string when concatenating them).