arch: arm: fix: pm_s2ram with CONFIG_DEBUG_THREAD_INFO

What is the change?
 - Fixes #83660 allowing device to now enter suspend mode even if
   CONFIG_DEBUG_THREAD_INFO is enabled.

Why is this needed?
 - z_sys_post_kernel was cleared as part of #d778d5c7
   to "allow debuggers to display the correct thread state after
   the first 3 instructions have run".
   This is not required while resuming from suspend and
   it prevents the device from entering suspend so,
   move it out of resume path.

Signed-off-by: Sudan Landge <sudan.landge@arm.com>