parisc: Reduce thread stack to 16 kb

In testing, I found that the thread stack can be 16 kB when using an irq
stack.  Without it, the thread stack needs to be 32 kB. Currently, the irq
stack is 32 kB. While it probably could be 16 kB, I would prefer to leave it
as is for safety.

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