ppc64 ifunc_wrapper add casts suggested by gcc warning.

vg_preloaded.c: In function ‘_vgnU_ifunc_wrapper’:
vg_preloaded.c:91:13: warning: assignment makes integer from pointer without a cast [enabled by default]

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14172 a5019735-40e9-0310-863c-91ae7b9d1cf9