convert gdb-remote 'A' launch to use LaunchProcess ()

This change modifies the 'A' command handler's launch code to launch
with LaunchProcess (). The net effect is that the default process
monitoring that LaunchProcess () adds will kick in, allowing the
GDBRemoteCommunicationServer to be able to reap processes started with
this facility correctly. Later, in the case of lldb-gdbserver, we'll
also have the proper process monitoring going on to really debug the
inferior process.