target-ppc: add vector count trailing zeros instructions

target-ppc: add vector count trailing zeros instructions

The following vector count trailing zeros instructions are
added from ISA 3.0.

vctzb - Vector Count Trailing Zeros Byte
vctzh - Vector Count Trailing Zeros Halfword
vctzw - Vector Count Trailing Zeros Word
vctzd - Vector Count Trailing Zeros Doubleword

Signed-off-by: Rajalakshmi Srinivasaraghavan <raji@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>