[ORC] Extract and tidy up JITTargetMachineBuilder, add unit test.

(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.

llvm-svn: 343423