systemclock: Reorganize defined checks for parts of GstSystemClock
The gst_system_clock_get_internal_time and gst_system_clock_get_resolution functions had some nested defined checks making this code somewhat harder to reason about and much harder to change. The logical meaning of the checks has changed but the actual code coming out of the pre-processor should not have changed significantly. The main logical change was flattening the checks for existence of posix timing functionality, from what I can tell these functions aren't available on Windows where they were trying to be included. I have checked the Linux and macOS output and they are functionally unchanged. Part-of: <gstreamer/gstreamer!829>