Ensure LpGBT sync FIFO are filled before reading them out

[Based on top of !192 (merged).]

As the title says, this merge ensures that the LpGBT sync FIFO are filled before reading them out. Additionally, the FIFO filling status is now reported in the backend registers for debugging purposes.

This change was required for the ME0 stack in the GIF++ to be fully stable. Some LpGBT links could previously unlock for a few BX with no other visible error than a RX_HAD_UNDERFLOW error. Since the stack was previously perfectly stable with the same fibers and backend, it is suspected that the GIF++ conditions increased a tiny bit the jitter in the RX recovered clock, leading to the sync FIFO to fully empty.

More details in the commit messages.