[NANO130] Fix optimization error with NVIC_SetVector/NVIC_GetVector on ARMC6

On ARMC6 with optimization level "-Os", the two functions NVIC_SetVector/NVIC_GetVector
will be optimized out. Add "volatile" qualifier to suppress it.

Author: ccli8

targets/TARGET_NUVOTON/TARGET_NANO100/device/cmsis_nvic.c

@@ -16,6 +16,13 @@
 #include "cmsis_nvic.h"
 #include "platform/mbed_error.h"
 
+/* Fix optimization error with NVIC_SetVector/NVIC_GetVector on ARMC6
+ *
+ * On ARMC6 with optimization level "-Os", the two functions NVIC_SetVector/
+ * NVIC_GetVector will be optimized out. Add "volatile" qualifier to suppress
+ * it.
+ */
+
 void NVIC_SetVector(IRQn_Type IRQn, uint32_t vector)
 {
     // NOTE: On NANO130, relocating vector table is not supported due to just 16KB small SRAM.
@@ -28,7 +35,7 @@ void NVIC_SetVector(IRQn_Type IRQn, uint32_t vector)
 
 uint32_t NVIC_GetVector(IRQn_Type IRQn)
 {
-    uint32_t *vectors = (uint32_t*) NVIC_FLASH_VECTOR_ADDRESS;
+    volatile uint32_t *vectors = (volatile uint32_t *) NVIC_FLASH_VECTOR_ADDRESS;
 
     // Return the vector
     return vectors[IRQn + 16];