As of February 2019 there is now an official project from Espressif to add the Xtensa chip architecture to LLVM.
For the discussion of this see this forum thread.
The repository that contains the Xtensa fork of LLVM is located at https://github.com/espressif/llvm-project
It is not yet in a usable state, but once it is we will start the process of supporting it in TinyGo.