Fixes the build for --disable-tcg.
This header is only needed for cross-hosting. Without CONFIG_TCG,
we know this is an AArch64 host, CONFIG_ATOMIC64 will be set, and
the TCG_OVERSIZED_GUEST block will never be compiled.
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>