tests: arm: sw_vector_table: fix build on many non systick platforms

In 4c93fcd35b, the default use case has been changed to support setting
a custom handler, but it's written in a way to only support MCUX and
SYSTICK based platforms. For any other platform the build break because
of undefined TIMER_IRQ_HANDLER and TIMER_IRQ_NUM.

Fix the conditional so that the custom timer line is only entered if a
custom handler is defined. The test would probably stil not run on those
platforms until a custom case is defined pointing at the custom timer
interrput handler, but at least it won't break CI.

Also drop a closing endif comment as that clearly became misleading.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

tests/arch/arm/arm_irq_vector_table/src/arm_irq_vector_table.c

@@ -257,11 +257,12 @@ const vth __irq_vector_table _irq_vector_table[IRQ_VECTOR_TABLE_SIZE] = {
 	[_ISR_OFFSET] = isr0,
 	[_ISR_OFFSET + 1] = isr1,
 	[_ISR_OFFSET + 2] = isr2,
-#ifndef CONFIG_CORTEX_M_SYSTICK
+#if defined(TIMER_IRQ_HANDLER) && !defined(CONFIG_CORTEX_M_SYSTICK)
 	[TIMER_IRQ_NUM] = TIMER_IRQ_HANDLER,
 #endif
 };
-#endif /* CONFIG_SOC_FAMILY_NORDIC_NRF */
+
+#endif
 
 /**
  * @}