kernel: thread: Allow stack in coherent memory

When allowing dynamic thread stack allocation the stack may come from
the heap in coherent memory, trying to use cached memory is over
complicated because of heap meta data and cache line sizes.
Also when userspace is enabled, stacks have to be page aligned and the
address of the stack is used to track kernel objects.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>