update stm32f20x linker script: ensure last block HEAP.

bsp/stm32f20x/stm32_rom.icf

@@ -28,8 +28,8 @@ do not initialize  { section .noinit };
 
 keep { section FSymTab };
 keep { section VSymTab };
+keep { section .rti_fn* };
 place at address mem:__ICFEDIT_intvec_start__ { readonly section .intvec };
 
 place in ROM_region   { readonly, block RTT_INIT_FUNC };
-place in RAM_region   { readwrite,
-                        block CSTACK, block HEAP };
+place in RAM_region   { readwrite, block CSTACK, last block HEAP};