exec: Use uintptr_t for guest_base

exec: Use uintptr_t for guest_base

This is more descriptive than 'unsigned long'.
No functional change, since these match on all linux+bsd hosts.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210212184902.1251044-4-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>