Fix libFuzzer build issue with -lstdc++ on Darwin

[IanPudney]

Summary

I can't confirm that there aren't other platforms with this issue.

[morehouse]

I reverted the other patch, so please update the other review rather than starting this new one.

compiler-rt/lib/fuzzer/CMakeLists.txt

if(NOT ${arch} MATCHES "i386") # i386 unsupported for .so version.
       )
     endif()
   endforeach()
 else()
 
 set(LIBFUZZER_SHARED_LINK_FLAGS ${SANITIZER_COMMON_LINK_FLAGS})
 
 set(LIBFUZZER_SHARED_LINK_LIBS ${SANITIZER_COMMON_LINK_LIBS})
+if (DARWIN)
+  list(APPEND LIBFUZZER_SHARED_LINK_LIBS "-lc++")
+else()
   list(APPEND LIBFUZZER_SHARED_LINK_LIBS "-lstdc++")
+endif()
 list(APPEND LIBFUZZER_SHARED_LINK_LIBS "-lm")
 list(APPEND LIBFUZZER_SHARED_LINK_LIBS "-lpthread")
 
 # If we aren't statically linking libc++ into the fuzzer, we can build the shared object directly
 add_compiler_rt_runtime(clang_rt.fuzzer_no_main
   SHARED
   OS ${FUZZER_SUPPORTED_OS}
   ARCHS ${FUZZER_SUPPORTED_ARCH}