Fix gdb.mi/mi-stack.exp when gcc generates a stack protector

Fix gdb.mi/mi-stack.exp when gcc generates a stack protector

I see some failures in the gdb.mi/mi-stack.exp test.  The test runs to
the callee4 function:

  int callee4 (void)
  {
    int A=1;
    int B=2;
    int C;
    int D[3] = {0, 1, 2};

    C = A + B;
    return 0;
  }

and expects to be stopped at the A=1 line.  However, when gcc generates
some stack protection code, it will stop at the { instead, as shown by
this disassembly (after I did "break callee4" and "run"):

  (gdb) disassemble /s
  Dump of assembler code for function callee4:
  /home/simark/src/binutils-gdb/gdb/testsuite/gdb.mi/mi-stack.c:
  26	{
     0x00005555555546ca <+0>:	push   %rbp
     0x00005555555546cb <+1>:	mov    %rsp,%rbp
     0x00005555555546ce <+4>:	sub    $0x20,%rsp
  => 0x00005555555546d2 <+8>:	mov    %fs:0x28,%rax
     0x00005555555546db <+17>:	mov    %rax,-0x8(%rbp)
     0x00005555555546df <+21>:	xor    %eax,%eax

  27	  int A=1; /* callee4 begin */
     0x00005555555546e1 <+23>:	movl   $0x1,-0x20(%rbp)

  28	  int B=2;
     0x00005555555546e8 <+30>:	movl   $0x2,-0x1c(%rbp)

The rest of the test relies on execution stopping on the A=1, so many things
fail after that.  This patch uses mi_continue_to_line instead, to stop at the
A=1 line precisely.

gdb/testsuite/ChangeLog:

	* gdb.mi/mi-stack.exp (test_stack_frame_listing): Use
	mi_continue_to_line.
	* gdb.mi/mi-stack.c (callee4): Add comment.