parisc: Make user stack size configurable

On parisc we need to initialize the memory layout for the user stack at
process start time to a fixed size, which up until now was limited to
the size as given by CONFIG_MAX_STACK_SIZE_MB at compile time.

This hard limit was too small and showed problems when compiling
ruby2.7, qmlcachegen and some Qt packages.

This patch changes two things:
a) It increases the default maximum stack size to 100MB.
b) Users can modify the stack hard limit size with ulimit and then newly
   forked processes will use the given stack size which can even be bigger
   than the default 100MB.

Reported-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>