EV_COG_AD4050LZ: keep IVT_NAME/blank_checksum symbols in LTO builds

Add a "used" attribute to IVT_NAME/blank_checksum
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, means that code must be emitted
for the function even if it appears that the function is not referenced.

targets/TARGET_Analog_Devices/TARGET_ADUCM4X50/TARGET_ADUCM4050/TARGET_EV_COG_AD4050LZ/device/startup_ADuCM4050.c

@@ -60,8 +60,10 @@ extern void SramInit(void);
   Checksum options
  *----------------------------------------------------------------------------*/
 
-#if defined(__ICCARM__)
+#if defined( __ICCARM__)
 __root
+#else
+__attribute__((used))
 #endif
 const uint32_t SECTION_PLACE(blank_checksum[],".checksum") =
 {
@@ -144,6 +146,11 @@ WEAK_FUNCTION( Root_Clk_Err_Handler       )
 /*----------------------------------------------------------------------------
   Exception / Interrupt Vector table
  *----------------------------------------------------------------------------*/
+#if defined( __ICCARM__)
+__root
+#else
+__attribute__((used))
+#endif
 const pFunc SECTION_PLACE(IVT_NAME[104],VECTOR_SECTION) = {
     (pFunc) INITIAL_SP,    /* Initial Stack Pointer */
     ADUCM4050_VECTORS