Move PPC's getDataLayoutString out of line and document it better.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196987 91177308-0d34-0410-b5e6-96231b3b80d8
Prune redundant dependencies in LLVMBuild.txt.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196988 91177308-0d34-0410-b5e6-96231b3b80d8