systemclock: Reorganize defined checks for parts of GstSystemClock

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: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/829>