mtext_rchr(3m17n) on Linux
The text b7b923d60da1, as Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie), Debian 7 (Wheezy) shipped it.
Return the position of the last occurrence of a character in an M-text.