- Fix header guard name.
- Define `__STDC_WANT_LIB_EXT1__` in `hdr/types/rsize_t.h` so that the
type is available for internal use.
- In accordance to the standard, make `stdio.h`, `stdlib.h`, `string.h`,
`time.h` and `wchar.h` define the type. It should already be available
in `stddef.h` as it's provided by Clang, not LLVM libc.