When compiling using toolchain_build_pnacl.py, we get errors of form:

  Don't use default labels in fully covered switches over enumerations

I tried different combinations of -Wno-covered-switch-default and
-Wno-error=covered-switch-default, but was not able to stop this
error from being generated. Hence, taking the simplier route of
removing -Werror from Makefile.

(see www.llvm.org/docs/CodingStandards.html for more details)

BUG=None
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/686103006