s390: Replace IS_ENABLED(EXPOLINE_*) with IS_ENABLED(CONFIG_EXPOLINE_*)

I've accidentally stumbled upon the IS_ENABLED(EXPOLINE_*) lines, which
obviously always evaluate to false. Fix this.

Fixes: f19fbd5e ("s390: introduce execute-trampolines for branches")
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>