clang fails freestanding test at `-O0` because it assumes `memcpy/memset` calls

## Problem

Doing:
```
make CC=clang  test && nm -u build/freestanding
```

Gives:
```
...
cc -std=c99 -Wextra -Wall -Werror -O0 -I. -ffreestanding -c test/freestanding.c -o build/freestanding
...


                 U memcpy
                 U memset
```

For some reason it assumes those calls. `gcc` doesn't.