S390: Place "s390:31-bit" after default arch in 64-bit arch list

S390: Place "s390:31-bit" after default arch in 64-bit arch list

On 64-bit platforms GDB did not include "s390:31-bit" in its list of
architecture names.  This patch fixes that.

To determine the list of architecture names for S390,
gdbarch_printable_names() walks through the linked list of BFD arches
starting with the default S390 arch, which is "s390:64-bit" on 64-bit
platforms.  But since "s390:64-bit" was at the end of that list, the
31-bit architecture was not reached.  The patch swaps the elements of
that list on 64-bit platforms.

bfd/ChangeLog:

	* cpu-s390.c (N): New macro.
	(bfd_s390_31_arch): New.  Define only if default target word size
	is 64 bits.  Otherwise define...
	(bfd_390_64_arch): ...this.  Make static.
	(bfd_s390_arch): Define according to the default target word size.
	Let the 'next' field point to the alternate arch.