s390/boot: Do not assume the decompressor range is reserved

s390/boot: Do not assume the decompressor range is reserved

When allocating a random memory range for .amode31 sections
the minimal randomization address is 0. That does not lead
to a possible overlap with the decompressor image (which also
starts from 0) since by that time the image range is already
reserved.

Do not assume the decompressor range is reserved and always
provide the minimal randomization address for .amode31
sections beyond the decompressor. That is a prerequisite
for moving the lowcore memory address from NULL elsewhere.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>