On the parisc architecture the stack grows upwards.
Move the TASK_UNMAPPED_BASE to high memory area as it's done by the
kernel on physical machines.

Signed-off-by: Helge Deller <deller@gmx.de>
Message-Id: <20220918194555.83535-9-deller@gmx.de>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>