Enable sys_getpriority and sys_setpriority for ppc64.

Bug 331830 - ppc64: WARNING: unhandled syscall: 96/97
(Miroslav Franc, mfranc@redhat.com)

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13861 a5019735-40e9-0310-863c-91ae7b9d1cf9