[PPC] Use alias symbols in address computation.

This seems to match what gcc does for ppc and what every other llvm
backend does.

This is a fixed version of r209638. The difference is to avoid any change
in behavior for functions. The logic for using constant pools for function
addresseses is spread over a few places and we have to keep them in sync.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209821 91177308-0d34-0410-b5e6-96231b3b80d8