[libc] Implement alphasort in dirent (#226487)

Implement the standard POSIX.1-2008 / POSIX.1-2024 function `alphasort`
in `<dirent.h>`, bringing functions declared in `dirent.yaml` to 100%
completion.

Fixes #226457

GitOrigin-RevId: 0206846e12b5d9126758112aa9e410fe0c69c6b6