target/ppc: Implement hashst and hashchk

Implementation for instructions hashst and hashchk, which were added
in Power ISA 3.1B.

It was decided to implement the hash algorithm from ground up in this
patch exactly as described in Power ISA.

Signed-off-by: Víctor Colombo <victor.colombo@eldorado.org.br>
Reviewed-by: Lucas Mateus Castro <lucas.araujo@eldorado.org.br>
Message-Id: <20220715205439.161110-3-victor.colombo@eldorado.org.br>
[danielhb: fix block comment in excp_helper.c]
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>