[S390] add __ucmpdi2() helper function

Provide __ucmpdi2() helper function on 31 bit so we don't run
again and again in compile errors like this one:

kernel/built-in.o: In function `T.689':
perf_counter.c:(.text+0x56c86): undefined reference to `__ucmpdi2'

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