[gdb] Move ChangeLog entries to their right files

I spotted a few misplaced entries in the ChangeLog-2019 entries, and
went on to fix them.

Looking around I saw a good number of other entries in other years.
Then OCD got the best of me and I fixed them all.

Also fixes cases of wrong paths in entries, like "* gdb/foo.c" instead
of "* foo.c".