[CMake] pass -Wno-pedantic to cc_kext builtins build

We don't pass -pedantic to the builtin builds in the makefiles, so we shouldn't do it here. This is a temporary fix for the broken bot:

http://lab.llvm.org:8080/green/job/clang-stage1-cmake-RA_build/7526/warnings8Result/new/

A more correct solution is in the works.

llvm-svn: 248515