m68k: Calculate THREAD_SIZE from THREAD_SIZE_ORDER

Current THREAD_SIZE_ORDER implementation is not generic.

Improve it by:
  - Defining THREAD_SIZE_ORDER based on the specific platform config,
  - Calculating THREAD_SIZE from THREAD_SIZE_ORDER.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Dawei Li <dawei.li@shingroup.cn>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20240304085455.125063-1-dawei.li@shingroup.cn


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>