Revert "llvm-c: Make target initializer functions external functions in lib."

This reverts commit r192316. The original change introduced circular
dependencies between libTarget and backends. That would broke a build unless
link everything into one big binary.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192329 91177308-0d34-0410-b5e6-96231b3b80d8