linux: use fd_to_filename instead of _fitoa_word in ttyname_r.

Simplifies the logic and makes intent clearer, while at the same time
decreasing binary size.

On x86_64, dropped from 1883270 to 1883206, a 64 byte decrease.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>