Use -Werror in external/python/cpython2

* Keep existing invalid-source-encoding and macro-redefined
  warnings to fix later.
* No -Werror for darwin_x86_64 yet. Must fix/suppress all warnings first.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I7e6e1974af3ab8f763baa3b3fdbe7ae11e841844
1 file changed