gcc_struct is for x86 only, and it generates an warning on ARM64 Clang/MinGW targets.

Signed-off-by: Cao Jiaxi <driver1998@foxmail.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20190503003618.10089-1-driver1998@foxmail.com
[PMM: dropped the slirp change as slirp is now a submodule]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>