(1) Adds comments for the API.
(2) Removes the setArch method: This is redundant: the setArchStr method on the
triple should be used instead.
(3) Turns EmulatedTLS on by default. This matches EngineBuilder's behavior.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343423 91177308-0d34-0410-b5e6-96231b3b80d8