Use lit.util.executeCommand instead of our own version

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@222718 91177308-0d34-0410-b5e6-96231b3b80d8
Fix comment on end of #endif to match #if

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@222719 91177308-0d34-0410-b5e6-96231b3b80d8