NUMAKER_M252KG: keep __vector_handlers symbol in LTO builds

Add a "used" attribute to __vector_handlers to fix ARMC6 build with
the "-flto" flag.
(Error: L6236E: No section matches selector - no section to be FIRST/LAST. )

This attribute, attached to a function/variable, means that code must be emitted
for the function even if it appears that the function is not referenced.

targets/TARGET_NUVOTON/TARGET_M251/device/startup_M251.c

@@ -155,7 +155,7 @@ WEAK_ALIAS_FUNC(OPA_IRQHandler, Default_Handler)        // 62: OPA Interrupt
 
 /* Vector table */
 #if defined(__ARMCC_VERSION)
-__attribute__ ((section("RESET")))
+__attribute__ ((section("RESET"), used))
 const uint32_t __vector_handlers[] = {
 #elif defined(__ICCARM__)
 extern uint32_t CSTACK$$Limit;