CUSTOM_EXIT_STUBS does not build

Not sure if this is still supported:

debug/64-bit build:

```
dynamorio/src/core/emit.c:582:48: error: passing argument 2 of ‘instr_set_note’ makes pointer from integer without a cast [-Werror=int-conversion]
instr_set_note(in, offset);
```

and

```
dynamorio/src/core/arch/instr_inline.h:411:1: note: expected ‘void *’ but argument is of type ‘uint {aka unsigned int}’
instr_set_note(instr_t *instr, void *value)
^~~~~~~~~~~~~~
cc1: all warnings being treated as errors
```