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.

llvm-svn: 352411
GitOrigin-RevId: 12f4b86808bf0874e222605c93c6e2be56b81bf5
1 file changed