Fix malloc thread step-out test on FreeBSD

After hitting the malloc() breakpoint on FreeBSD our top frame is actually
an inlined function malloc_init.

  * frame #0: 0x0000000800dcba19 libc.so.7`malloc [inlined] malloc_init at malloc.c:5397
    frame #1: 0x0000000800dcba19 libc.so.7`malloc(size=1024) + 9 at malloc.c:5949
    frame #2: 0x00000000004006e5 test_step_out_of_malloc_into_function_b_with_dwarf`b(val=1) + 37 at main2.cpp:29

Add a heuristic to keep stepping out until we come to a non-malloc caller,
before checking if it is our desired caller from the test code.

llvm.org/pr17944