signal/all: remove return value from copy_siginfo_to_user

signal/all: remove return value from copy_siginfo_to_user

Since copy_siginfo_to_user always returns 0, make it void
and remove any checks for return value from calling functions.

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>