s390: limit brk randomization to 32MB

For a 64-bit process the randomization of the program break is quite
large with 1GB. That is as big as the randomization of the anonymous
mapping base, for a test case started with '/lib/ld64.so.1 <exec>'
it can happen that the heap is placed after the stack. To avoid
this limit the program break randomization to 32MB for 64-bit and
keep 8MB for 31-bit.

Reported-by: Stefan Liebler <stli@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>