[bsp][ch32v307v] add _head_end for link file

bsp/wch/risc-v/ch32v307v-r1/board/linker_scripts/link.lds

@@ -198,6 +198,7 @@ SECTIONS
 
     .stack ORIGIN(RAM) + LENGTH(RAM) - __stack_size :
     {
+        PROVIDE( _heap_end = . );
         . = ALIGN(4);
         PROVIDE(_susrstack = . );
         . = . + __stack_size;