Workaround libc++ defining __cpp_lib_hardware_interference_size

Workaround libc++ defining __cpp_lib_hardware_interference_size

Older versions of libc++ define the feature test macro
__cpp_lib_hardware_interference_size, but not the
std::hardware_destructive_interference_size and
std::hardware_constructive_interference_size variables it's for. Not sure which
version fixed it, so don't use it at all with libc++ for now.