target/ppc: Add popcntb instruction to POWER5+ processors

popcntb instruction was added in ISA v2.02. Add support for POWER5+
processors since they implement ISA v2.03.

PPC970 CPUs implement v2.01 and do not support popcntb.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>
Message-Id: <20220105095142.3990430-2-clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>