Big-endian hosts: Fix "set architecture cris"

The all-architectures-1.exp test case currently yields 66 FAILs on s390x,
because the "set architecture" command fails each time when attempting to
switch to "cris", "crisv32", or "cris:common_v10_v32".  Actually, the
command would succeed if the endianness had been set to "little" before.
Instead, the test case sets the endianness to "auto", which results in
"big" on s390x.

So on x86_64:

  (gdb) set endian auto
  The target endianness is set automatically (currently little endian)
  (gdb) set architecture cris
  warning: A handler for the OS ABI "AIX" is not built into this configuration
  of GDB.  Attempting to continue with the default cris settings.

  The target architecture is assumed to be cris

But on s390x:

  (gdb) set endian auto
  The target endianness is set automatically (currently big endian)
  (gdb) set architecture cris
  Architecture `cris' not recognized.

See also the test results for s390x and ppc64be:

  https://sourceware.org/ml/gdb-testers/2016-q4/msg05150.html
  https://sourceware.org/ml/gdb-testers/2016-q4/msg05713.html

Indeed, cris_gdbarch_init in cris-tdep.c returns a failure unless the
user-specified endianness is "little".  Other architectures usually ignore
the user-specified endianness and return a valid gdbarch anyhow, even if
they can not really cope with the given endianness.

This patch removes the check in cris-tdep.c and forces little-endian byte
order instead.

gdb/ChangeLog:

	* cris-tdep.c (cris_gdbarch_init): Remove check for
	info.byte_order and force it to BFD_ENDIAN_LITTLE.