NUMAKER_IOT_M263A: 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.

Author: maciejbocianski

targets/TARGET_NUVOTON/TARGET_M261/device/startup_M261.c

@@ -193,7 +193,7 @@ WEAK_ALIAS_FUNC(TRNG_IRQHandler, Default_Handler)        // 101:
 
 /* Vector table */
 #if defined(__CC_ARM) || (defined(__ARMCC_VERSION) && (__ARMCC_VERSION >= 6010050))
-__attribute__ ((section("RESET")))
+__attribute__ ((section("RESET"), used))
 const uint32_t __vector_handlers[] = {
 #elif defined(__ICCARM__)
 extern uint32_t CSTACK$$Limit;