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/compiler-rt/trunk@352411 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/config-ix.cmake

@@ -12,7 +12,7 @@ function(check_linker_flag flag out_var)
   cmake_pop_check_state()
 endfunction()
 
-check_library_exists(c __libc_start_main "" COMPILER_RT_HAS_LIBC)
+check_library_exists(c fopen "" COMPILER_RT_HAS_LIBC)
 if (COMPILER_RT_USE_BUILTINS_LIBRARY)
   include(HandleCompilerRT)
   find_compiler_rt_library(builtins COMPILER_RT_BUILTINS_LIBRARY)