[compiler-rt][Fuzzer] Use the new C++ ABI namespace CMake support

[phosek]

Summary

libc++ now supports customizing the ABI namespace directly from the
CMake build so we no longer need to rely on custom CFLAGS.

compiler-rt/lib/fuzzer/CMakeLists.txt

macro(partially_link_libcxx name dir arch)
     )
   endmacro()
 
   foreach(arch ${FUZZER_SUPPORTED_ARCH})
     get_target_flags_for_arch(${arch} TARGET_CFLAGS)
     set(LIBCXX_${arch}_PREFIX ${CMAKE_CURRENT_BINARY_DIR}/libcxx_fuzzer_${arch})
     add_custom_libcxx(libcxx_fuzzer_${arch} ${LIBCXX_${arch}_PREFIX}
       CFLAGS ${TARGET_CFLAGS}
-             -D_LIBCPP_ABI_VERSION=Fuzzer
              -D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS=1
              -fvisibility=hidden
       CMAKE_ARGS -DCMAKE_CXX_COMPILER_WORKS=ON
                  -DLIBCXX_ENABLE_EXCEPTIONS=OFF
                  -DLIBCXX_ENABLE_SHARED=OFF
+                 -DLIBCXX_ABI_NAMESPACE=Fuzzer
                  -DLIBCXX_CXX_ABI=none)
     target_compile_options(RTfuzzer.${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()
 endif()
 
 if(COMPILER_RT_INCLUDE_TESTS)
   add_subdirectory(tests)
 endif()