gdbstub: modernise DEBUG_GDB

Convert the a gdb_debug helper which compiles away to nothing when not
used but still ensures the format strings are checked. There is some
minor code motion for the incorrect checksum message to report it
before we attempt to send the reply.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <20170712105216.747-2-alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
