s390: valgrind side support for PFPO. New hwcap added.
See companion patch VEX r2719.

Patch by Maran Pakkirisamy (maranp@linux.vnet.ibm.com).
Part of fixing BZ #307113


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13387 a5019735-40e9-0310-863c-91ae7b9d1cf9