[Fuzzer] When building for Fuchsia, add an explicit libc++ dependency

[phosek]

Summary

libFuzzer dependes on C++ library, when building for Fuchsia, we
need to ensure that libFuzzer is only being built after libc++
has been built, so we add an explicity dependency on it.

[morehouse]

LGTM

compiler-rt/trunk/lib/fuzzer/CMakeLists.txt

add_custom_libcxx(libcxx_fuzzer_${arch} ${LIBCXX_${arch}_PREFIX}
                  -DLIBCXX_CXX_ABI=none)
     target_compile_options(RTfuzzer.${arch} PRIVATE -isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
     add_dependencies(RTfuzzer.${arch} libcxx_fuzzer_${arch}-build)
     target_compile_options(RTfuzzer_main.${arch} PRIVATE -isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
     add_dependencies(RTfuzzer_main.${arch} libcxx_fuzzer_${arch}-build)
     partially_link_libcxx(fuzzer_no_main ${LIBCXX_${arch}_PREFIX} ${arch})
     partially_link_libcxx(fuzzer ${LIBCXX_${arch}_PREFIX} ${arch})
   endforeach()
+elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Fuchsia" AND HAVE_LIBCXX)
+  foreach(arch ${FUZZER_SUPPORTED_ARCH})
+    add_dependencies(RTfuzzer.${arch} cxx)
+    add_dependencies(RTfuzzer_main.${arch} cxx)
+  endforeach()
 endif()
 
 if(COMPILER_RT_INCLUDE_TESTS)
   add_subdirectory(tests)
 endif()