[PowerPC] Fix readcyclecounter to be custom expanded for all 32-bit targets

We need to use the custom expansion of readcyclecounter on all 32-bit targets
(even those with 64-bit registers). This should fix the ppc64 buildbot.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223182 91177308-0d34-0410-b5e6-96231b3b80d8