Fix crash caused by log_timezone patch if we attempt to emit any elog messages

Fix crash caused by log_timezone patch if we attempt to emit any elog messages
between the setting of log_line_prefix and the setting of log_timezone.  We
can't realistically set log_timezone any earlier than we do now, so the best
behavior seems to be to use GMT zone if any timestamps are to be logged during
early startup.  Create a dummy zone variable with a minimal definition of GMT
(in particular it will never know about leap seconds), so that we can set it
up without reference to any external files.