scripts: elf_helper.py: fix stack declared K_THREAD_STACK_EXTERN

If a stack is declared with K_THREAD_STACK_EXTERN first, analyze array
in elf_helper.py will ignore this declaration which will be referenced
by the actual instances via the tag DW_AT_specification, so that this
stack can't be detected by the kernel object detection mechanism, and
this will cause userspace not work.

Fixes: #16760.

Signed-off-by: Wentong Wu <wentong.wu@intel.com>