armv6m: fix regresion from https://github.com/apache/nuttx/pull/14881

reason:
svc call may trigger hardfalt

Signed-off-by: hujun5 <hujun5@xiaomi.com>

arch/arm/src/armv6-m/arm_doirq.c

@@ -65,7 +65,7 @@ uint32_t *arm_doirq(int irq, uint32_t *regs)
    * is invalid, and we can safely overwrite it.
    */
 
-  if (!(NVIC_IRQ_SVCALL == irq && regs[REG_R0] == SYS_restore_context))
+  if (*running_task != NULL)
     {
       tcb->xcp.regs = regs;
     }

arch/arm/src/armv6-m/arm_sigdeliver.c

@@ -165,6 +165,7 @@ retry:
   rtcb->irqcount--;
 #endif
 
+  g_running_tasks[this_cpu()] = NULL;
   rtcb->xcp.regs = rtcb->xcp.saved_regs;
   arm_fullcontextrestore();
   UNUSED(regs);

arch/arm/src/common/arm_exit.c

@@ -60,9 +60,18 @@ void up_exit(int status)
 
   nxtask_exit();
 
-  /* Scheduler parameters will update inside syscall */
+  /* Update g_running_tasks */
 
+#ifdef CONFIG_ARCH_ARMV6M
+  /* ARMV6M syscal may trigger hard fault， We use
+   * running_task != NULL to determine whether it is
+   * a context for restoration.
+   */
+
+  g_running_tasks[this_cpu()] = NULL;
+#else
   g_running_tasks[this_cpu()] = this_task();
+#endif
 
   /* Then switch contexts */