Fix copyright years in gdbarch.h and gdbarch.c.

Fix copyright years in gdbarch.h and gdbarch.c.

copyright.py was updating gdbarch.sh' copyright years, but not
the function that genarates the copyright headers for gdbarch.[hc].

This patch makes the following changes:
  - fixes gdbarch.sh and regenerates gdbarch.[hc]

  - Modify copyright.py to remind the user to update gdbarch.sh
    by hand.  This should also remind the user to rerun the script
    and update gdbarch.[hc] - see next point;

  - Allow copyright.py to update gdbarch.[hc]'c copyright years,
    even though these are generated files;  This should free
    the user from having to update gdbarch.[hc] himself;

gdb/ChangeLog:

        * gdbarch.sh (copyright): Update copyright years.
        * gdbarch.h, gdbarch.c: Regenerate.
        * copyright.py (EXCLUDE_LIST): Remove gdbarch.h and gdbarch.c.
        (MULTIPLE_COPYRIGHT_HEADERS): Add 'gdb/gdbarch.sh'.