[stdlib] Set PYTHONIOENCODING when building the stdlib
Ubuntu 18.04 seems to be defaulting to 'ascii', causing an error when
using line-directive combined with the utf-8 diagnostic output used when
early swift syntax is enabled.
(cherry picked from commit afd3f172cbd3c4a9a1bbe8ba7d15efbf65e754f0)
(cherry picked from commit f0382808c8cc8ad7cdaeb7c0d152ce9b8c11d315)
