[S390] reduce miminum gap between stack and mmap_base

Reduce minimum gap between stack and mmap_base to 32MB. That way there
is a bit more space for heap and mmap for tight 31 bit address spaces.

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