s390: add CPU_BIG_ENDIAN config option

Make sure that s390 appears to be a big endian machine by defining
this config option.

Without this s390 appears to be little endian as seen by e.g. the
recordmount script: "perl ./scripts/recordmcount.pl "s390" "little"
"64""
This has no practical impact within the script since the endian
variable is only evaluated for mips. However there are already a
couple of common code places which evaluate this config option. None
of them is relevant for s390 currently though.

To avoid any issues in the future (and fix the recordmcount oddity)
add the new config option.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>