[libc]: Clean up unnecessary function pointers in scanf (#121215)
Resolves #115394
1. Move definitions of cross-platform `getc` `ungetc` to `reader.h`.
2. Remove function pointer members to define them once per platform in
`.h`
3. Built in overlay mode in macOS m1
4. Remove `reader.cpp` as it's empty now
Also, full build doesn't yet build on macos m1 AFAIK