kernel: align thread stack size declaration

When thread stack is defined as an array, K_THREAD_STACK_LEN()
is used to calculate the size for each stack in the array.
However, standalone thread stack has its size calculated by
Z_THREAD_STACK_SIZE_ADJUST() instead. Depending on the arch
alignment requirement, they may not be the same... which
could cause some confusions. So align them both to use
K_THREAD_STACK_LEN().

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