s390: revert ELF_ET_DYN_BASE base changes

This reverts commit a73dc537.

Reducing the base address for 31-bit PIE executables from
(STACK_TOP/3)*2 to 4MB broke several compat programs which
use -fpie to move the executable out of the lower 16MB.

Cc: <stable@vger.kernel.org> # 4.13+
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>