[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".