CMake: Enable -Werror for the -fno-function-sections check

If the -fno-function-sections flag isn't recognized, the compiler
emits a warning. This isn't enough to cause the check to fail, so we
enable -Werror as well.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@200753 91177308-0d34-0410-b5e6-96231b3b80d8