clang doesn't support the __noclone__ attribute and emits a warning about
it. Fortunately clang also implements a mechanism for asking if a particular
attribute is implemented; use it. We assume that if the compiler doesn't
support __has_attribute() then it must be GCC and must support __noclone__.
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>