[CRIS] Don't internal error if forced big endian

[CRIS] Don't internal error if forced big endian

This fixes:

  $ ./gdb -q -ex "set endian big" -ex "set architecture cris"
  The target is assumed to be big endian
  .../src/gdb/cris-tdep.c:4051: internal-error: cris_gdbarch_init: big endian byte order in info
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.
  Quit this debugging session? (y or n)

The "set cris-version" command can likewise cause internal errors.

The gdbarch init routine should be returning 0 to reject the
architecture instead of internal erroring on user input.

gdb/ChangeLog:
2016-03-09  Pedro Alves  <palves@redhat.com>

	* cris-tdep.c (cris_gdbarch_init): Return 0 if the info's byte
	order is BFD_ENDIAN_BIG or if the cris version is unsupported.