Now that ChangeLog entries are no longer used for sim patches,
this commit renames all relevant sim ChangeLog to ChangeLog-2021,
similar to what we would do in the context of the "Start of New
Year" procedure.
The purpose of this change is to avoid people merging ChangeLog
entries by mistake when applying existing commits that they are
currently working on.
Also throw in a .gitignore entry to keep people from adding new
ChangeLog files anywhere in the sim tree.
Provide a simple example simulator for people porting to new targets
to use as a reference. This one has the advantage of being used by
people and having a fun program available for it.
It doesn't require a special target -- the example simulators can be
built for any existing port.