message: Add note about ptxas bug affecting XLA:GPU to relnotes.
diff: * Starting from 1.6 release, our prebuilt binaries will use AVX instructions.
  This may break TF on older CPUs.
  +## Known Bugs
  +* Using XLA:GPU with CUDA 9 and CUDA 9.1 results in garbage results and/or
  + `CUDA_ILLEGAL_ADDRESS` failures.
  +
  + Google discovered in mid-December 2017 that the PTX-to-SASS compiler in CUDA 9
  + and CUDA 9.1 sometimes does not properly compute the carry bit when
  + decomposing 64-bit address calculations with large offsets (e.g. `load [x +
  + large_constant]`) into 32-bit arithmetic in SASS.
  +
  + As a result, these versions of `ptxas` miscompile most XLA programs which use
  + more than 4GB of temp memory. This results in garbage results and/or
  + `CUDA_ERROR_ILLEGAL_ADDRESS` failures.
  +
  + A fix in CUDA 9.1.121 is expected in late February 2018. We do not expect a
  + fix for CUDA 9.0.x. Until the fix is available, the only workaround is to
  + [downgrade](https://developer.nvidia.com/cuda-toolkit-archive) to CUDA 8.0.x
  + or disable XLA:GPU.
  +
  + TensorFlow will print a warning if you use XLA:GPU with a known-bad version of
  + CUDA; see e00ba24c4038e7644da417ddc639169b6ea59122.
  +
  ## Major Features And Improvements
  * [Eager execution](https://github.com/tensorflow/tensorflow/tree/r1.5/tensorflow/contrib/eager)
  preview version is now available.
  