[libc] Define Annex K's errno_t in specified headers (#187700)

- Change `errno.h.def` to include a placeholder where hdrgen emits the
public API, which contains the `errno_t` definition.
- Make headers `stdio.h`, `stdlib.h`, `string.h` and `time.h` also
define `errno_t` as specified in the standard.

NOKEYCHECK=True
GitOrigin-RevId: 27ba9e2a44c11f8123528c350227db2c9a707c8f