kernel: return error instead of misaligned k_thread object

k_object_alloc(K_OBJ_THREAD) returns a usable struct k_thread pointer.
This pointer is 4 byte aligned. On x86 and x86_64 struct _thread_arch
has a member which requires alignment. Since this is currently not
supported k_object_alloc(K_OBJ_THREAD) now returns an error instead of
a misaligned pointer.

Signed-off-by: Maximilian Bachmann <m.bachmann@acontis.com>