ARM GCC Retarget dosen't work in coresight example

I have not seen any axi transfer when I run coresight example helloworld.

And the cpu program counter stopping in some code

I have some questions when I check my IMAGE0.lst file

1 .why printf is branch to putchar instead of printf in coresight example?

2. Is GCC retarget modify _write function?

3. What GCC glibc version recommended to use for coresight example and CPU-R8?

-------------IMAGE0.lst----------------

printf("A");

7d18:  e3a00041  mov r0,  #65  ; 0x41

7d1c:  fa0000a3    blx 7fb0   <putchar>

---------------------------------------------

--------------retarget.c-------------------

void output_char(int c){

    volatile int *tube = 0xfc000000;

    *tube = c;

}

int _write(int fh, char *buf, int count)  {

int i = 0;

while (i < count){

    output_char(buf[ i++ ]);

     }

}

-----------------------------------------

I use GCC 8.3.1 20190703 (release gun tool for arm embedded processor 8 2019.q3)  and glibc is 2.14