arm: userspace: fix initial user sp location

The initial user stack pointer gets rounded to STACK_ALIGN_SIZE,
which can be much finer granularity than MPU regions. With
certain stack size values passed to k_thread_create(), the stack
pointer can be set past the defined region for the user thread's
stack, causing an immediate MPU fault.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>