arm64: implement pmull{2}.


git-svn-id: svn://svn.valgrind.org/vex@2888 8f6e269a-dfd6-0310-a8e1-e2731360e62c
The vector versions of the count leading zeros/sign bits primops
(Iop_Cls* and Iop_Clz*) misleadingly imply a signedness in the
incoming lanes.  Rename them to fix this.  Fixes #326026.


git-svn-id: svn://svn.valgrind.org/vex@2889 8f6e269a-dfd6-0310-a8e1-e2731360e62c