NU_PFM_M2351_NPSA: 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_M2351/device/startup_M2351.c

@@ -192,7 +192,7 @@ WEAK_ALIAS_FUNC(TRNG_IRQHandler, Default_Handler)       // 101:
 
 /* Vector table */
 #if defined(__ARMCC_VERSION)
-__attribute__ ((section("RESET")))
+__attribute__ ((section("RESET"), used))
 const uint32_t __vector_handlers[] = {
 #elif defined(__ICCARM__)
 #if defined (__ARM_FEATURE_CMSE) && (__ARM_FEATURE_CMSE == 3U) && (TFM_LVL > 0)