Move copyright code from gdbarch.py to new file

This moves the copyright code from gdbarch.py to a new Python source
file, gdbcopyright.py.  The function in this file will find the
copyright dates by scanning the calling script.  This will be reused
in a future patch.

This involved minor changes to the output of gdbarch.py.  Also, I've
updated copyright.py to remove the reference to gdbarch.sh.  We don't
need to mention gdbarch.py there, either.