kernel: fix thread prio and stack size types in some APIs

Prio should be an int, since values are small integers, not a fixed-size
int32_t. It aligns with the prio parameters of the other APIs.

Stack size should be size_t.

Change-Id: Id29751b86c4ad7a7c2a7ffe446c2a96ae83c77bf
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>