vc4: Add a sentinel after simulator buffers for buffer overflow detection.

vc4: Add a sentinel after simulator buffers for buffer overflow detection.

This is a little bit like the mprotect-based fencing I've experimented
with, but it's simple and low overhead.  The downside is that only catches
writes, not reads.

It didn't catch any bad writes on a current piglit run, but may be useful
in the future.