kernel: add stack object alignment macro interface

A new macro Z_THREAD_STACK_OBJ_ALIGN() defines the alignment
of the lowest memory address of a stack object.

Related is a new arch interface ARCH_THREAD_STACK_OBJ_ALIGN()
which lets arches specifiy this. ARCH_STACK_PTR_ALIGN or a
power-of-two ceiling is used if not defined.

The default stack macros now use this instead of hard-coding
ARCH_STACK_PTR_ALIGN.

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