arch & sched: task: Fix up_exit() and nxtask_exit() for SMP

Summary:
- During repeating ostest with sabre-6quad:smp (QEMU),
  I noticed that pthread_rwlock_test sometimes stops
- Finally, I found that nxtask_exit() released a critical
  section too early before context switching which resulted in
  selecting inappropriate TCB
- This commit fixes this issue by moving nxsched_resume_scheduler()
  from nxtask_exit() to up_exit() and also removing
  spin_setbit() and spin_clrbit() from nxtask_exit()
  because the caller holds a critical section
- To be consistent with non-SMP cases, the above changes
  were done for all CPU architectures

Impact:
- This commit affects all CPU architectures regardless of SMP

Testing:
- Tested with ostest with the following configs
- sabre-6quad:smp (QEMU, dev board), sabre-6quad:nsh (QEMU)
- spresense:wifi_smp
- sim:smp, sim:ostest
- maix-bit:smp (QEMU)
- esp32-devkitc:smp (QEMU)
- lc823450-xgevk:rndis

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

arch/z80/src/common/z80_exit.c

@@ -152,6 +152,10 @@ void up_exit(int status)
   tcb = this_task();
   sinfo("New Active Task TCB=%p\n", tcb);
 
+  /* Adjusts time slice for SCHED_RR & SCHED_SPORADIC cases */
+
+  nxsched_resume_scheduler(tcb);
+
 #ifdef CONFIG_ARCH_ADDRENV
   /* Make sure that the address environment for the previously running
    * task is closed down gracefully (data caches dump, MMU flushed) and