cpu: Check that the memory system is in the correct mode

cpu: Check that the memory system is in the correct mode

This patch adds checks to all CPU models to make sure that the memory
system is in the correct mode at startup and when resuming after a
drain.  Previously, we only checked that the memory system was in the
right mode when resuming. This is inadequate since this is a
configuration error that should be detected at startup as well as when
resuming. Additionally, since the check was done using an assert, it
wasn't performed when NDEBUG was set (e.g., the fast target).