orconce: Fix typo in GCC __sync-based implementation

The __sync_val_compare_and_swap() function is defined as 1:

type __sync_val_compare_and_swap (type *ptr, type oldval, type newval, ...)

However, the pre-C11 orc_once_enter() function is calling the function like __sync_val_compare_and_swap(ptr, newval, oldval). This breaks the orconce implementation: the atomic variable is never set to the initialized state and so the initialization function is called every time orc_once_enter() is called.

This bug was originally discovered through Buildroot. GStreamer packages are currently build with -std=c99 or -std=gnu99 there. These flags disable the C11 orconce codepath and instead the old __sync codepath is selected. The ultimate effect of this was that all elements relying on ORC (e.g. videotestsrc, videoconvert) were slowed down significantly by repeated recompiling of the ORC programs.