fix(cros-workon.eclass): Do not add -clang to CFLAGS

Not sure how this managed to work in the past, perhaps the test for
using Google's common.mk used to always fail but now works properly?
In any case, we don't use clang any more.
Merge pull request #251 from marineam/cros-workon

fix(cros-workon.eclass): Do not add -clang to CFLAGS