[Sanitizer] fix signed/unsigned compare warnings in sanitizer_stacktrace_test. Build sanitizer_common tests with -Werror as we use predictable just-built Clang for building them

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@176014 91177308-0d34-0410-b5e6-96231b3b80d8