EV_COG_AD3029LZ: 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.

Author: maciejbocianski

targets/TARGET_Analog_Devices/TARGET_ADUCM302X/TARGET_ADUCM3029/TARGET_EV_COG_AD3029LZ/device/startup_ADuCM3029.c

@@ -62,7 +62,9 @@ extern void SramInit(void);
 
 #if defined( __ICCARM__)
 __root
-#endif /* __ICCARM__ */
+#else
+__attribute__((used))
+#endif
 const uint32_t SECTION_PLACE(blank_checksum[],".checksum") =
 {
     BLANKX60,BLANKX600
@@ -139,6 +141,11 @@ WEAK_FUNCTION( DMA_SIP7_Int_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 */