Build: maint: simplify changelog targets

Drop the "rc-changes" target. Instead, the "changelog" target will now do the
right thing depending on whether LAST_RELEASE contains an rc suffix.

This currently uses one giant printf, which avoids having to use a temporary
file, but we might have to change it one day if the command line grows too
long.

Compared to the previous implementation, this will preserve any uncommitted
changes in ChangeLog.