GCC 8 warns about alias with different types

Our aliases for some `*_chk` functions are rejected by GCC 8, because they have different types.

```
dynamorio/core/string.c:174:1: error: ‘__memmove_chk’ alias between functions of incompatible types ‘void *(void *, const void *, size_t, size_t)’ {aka ‘void *(void *, const void *, long unsigned int, long unsigned int)’} and ‘void *(void *, const void *, size_t)’ {aka ‘void *(void *, const void *, long unsigned int)’} [-Werror=attribute-alias]
__memmove_chk(void *dst, const void *src, size_t n, size_t dst_len)
^~~~~~~~~~~~~
dynamorio/core/string.c:153:1: note: aliased declaration here
memmove(void *dst, const void *src, size_t n)
^~~~~~~
```