linux-user: Zero out target_siginfo_t in force_sig()

The target_siginfo_t we populate in force_sig() will eventually
get copied onto the target's stack. Zero it out so that any extra
padding in the sifields union is consistently zero when the guest
sees it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210813131809.28655-5-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>