Update the cpu specified on some PPC regression tests

Some of these tests did not specify a cpu but were also sensitive to
instruction scheduling and/or register assignment choices. A few others
similarly-sensitive tests specified a cpu (often the POWER7), and while the P7
currently uses the default model for PPC64, this will soon change. For those
tests which should not really be cpu-dependent anyway, the cpu is set to the
generic 'ppc64'.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195977 91177308-0d34-0410-b5e6-96231b3b80d8