Replace cmake check for printf with a check for fopen.

Printf is a builtin, and the check fails with -Werror because of a clang
warning about an incompatible redeclaration.

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@255188 91177308-0d34-0410-b5e6-96231b3b80d8