Fix promotion in Ai-Wok

Fix promotion in Ai-Wok

When in a variant with fixed promotion, such as Makruk, the promotion
piece did not participate, this led to disappearence of the piece, and
an =. suffix in SAN. This is relevant for the Makruk variant Ai-Wok,
which replaces the Met by a much more powerful Ai-Wok, which could not
be represented by another XBoard piece type due to this problem.
Now we search through the pieceToCharTable until a valid piece is found,
which works when the Ai-Wok is represented by a SMIRF Archbishop symbol.