Revert "[CMake] Use __libc_start_main rather than fopen when checking for C library"

This reverts commit r352341: it broke the build on macOS which doesn't
seem to provide __libc_start_main in its C library.

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@352411 91177308-0d34-0410-b5e6-96231b3b80d8