kernel: userspace: reserve stack space to store local data

This enables reserving little space on the top of stack to store
data local to thread when CONFIG_USERSPACE. The first customer
of this is errno.

Note that ARC, due to how it lays out the user stack and
privilege stack, sets the pointer itself rather than
relying on the common way.

Fixes: #9067

Signed-off-by: Daniel Leung <daniel.leung@intel.com>