Turn on the STM32F4's flash cache and prefetcher.
Turn on the STM32F4's flash cache and prefetcher.

This does a surprisingly good job of hiding jitter, particularly
considering how simple it is.  But it's not a silver bullet:

- It only works on Z-rev parts.  The original batch of STM32F4 chips
  have broken prefetch units.  Wheeeee.

- Some of our lowest-latency Flash access patterns are very likely
  to miss the cache.  For example: vector table fetches during
  interrupt entry.

These caches are very simple, and we can't lock memory into them.
(Moreover, the cache is tiny.)  So more work needed.

cbiffle/m4vgalib-demos,cbiffle/m4vgalib-demos,cbiffle/m4vgalib-demos