no-systick targets: fix systick irq handler setup

For targets with no H/W systick (NO_SYSTICK), SysTick_Handler needs to be remapped to specific irq emulating systick.

Author: maciejbocianski

rtos/source/TARGET_CORTEX/mbed_rtx_idle.cpp

@@ -61,7 +61,7 @@ extern "C" {
         IRQn_Type irq = OsTimer::get_irq_number();
 
         NVIC_SetPriority(irq, 0xFF);
-#ifdef NVIC_RAM_VECTOR_ADDRESS
+#if defined(NVIC_RAM_VECTOR_ADDRESS) || defined(NO_SYSTICK)
         NVIC_SetVector(irq, (uint32_t)handler);
 #else
         MBED_ASSERT(handler == (IRQHandler_t)NVIC_GetVector(irq));