Add nullability specifiers to functions

_Nonnull
_Nullable

```
../include/stdlib.h:354:51: warning: pointer is missing a nullability type specifier (_Nonnull, _Nullable, or _Null_unspecified) [-Wnullability-completeness]
long long strtoll(const char* __restrict str, char** __restrict str_end, int base);
^
../include/stdlib.h:354:51: note: insert '_Nullable' if the pointer may be null
long long strtoll(const char* __restrict str, char** __restrict str_end, int base);
^
_Nullable
../include/stdlib.h:354:51: note: insert '_Nonnull' if the pointer should never be null
long long strtoll(const char* __restrict str, char** __restrict str_end, int base);
^
_Nonnull
```