s390: disable asm code expolines if cc does not support it

To avoid a mixture of asm code with expolines and c code without them,
propagate CC_USING_EXPOLINE to KBUILD_AFLAGS and use it to detect
whether asm code should have expolines or not.

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>