s390: fix linker script for 31 bit builds

commit c985cb37f1b39c2c8035af741a2a0b79f1fbaca7 upstream.

Because of a change in the s390 arch backend of binutils (commit 23ecd77
"Pick the default arch depending on the target size" in binutils repo)
31 bit builds will fail since the linker would now try to create 64 bit
binary output.
Fix this by setting OUTPUT_ARCH to s390:31-bit instead of s390.
Thanks to Andreas Krebbel for figuring out the issue.

Fixes this build error:

  LD      init/built-in.o
s390x-4.7.2-ld: s390:31-bit architecture of input file
 `arch/s390/kernel/head.o' is incompatible with s390:64-bit output

Cc: Andreas Krebbel <Andreas.Krebbel@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>