Construct the MCStreamer before constructing the MCTargetStreamer.

This has a few advantages:
* Only targets that use a MCTargetStreamer have to worry about it.
* There is never a MCTargetStreamer without a MCStreamer, so we can use a
  reference.
* A MCTargetStreamer can talk to the MCStreamer in its constructor.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200129 91177308-0d34-0410-b5e6-96231b3b80d8