[hdl] use new gc_sync for pulse synchroniser as well.

The main reason for doing this is so that all our sync modules are based directly on
gc_sync (instead of using it indirectly through gc_sync_ffs).

Another benefit of this is that the feedback loop of the pulse synchroniser will now be two clock
cycles shorter (one input clock cycle + one output clock cycle), since gc_sync_ffs is using one
more flip-flop compared to gc_sync.

This will also reduce the number of warnings in various synthesis and simulation tools, since
gc_pulse_syncrhonizer is also used by the gc_sync_word modules, as well as the async dual clock
FIFOs.

Signed-off-by: Dimitris Lampridis <dimitris.lampridis@cern.ch>