linux-user/nios2: Fixes for signal frame setup

linux-user/nios2: Fixes for signal frame setup

Do not confuse host and guest addresses.  Lock and unlock
the target_rt_sigframe structure in setup_rt_sigframe.

Since rt_setup_ucontext always returns 0, drop the return
value entirely.  This eliminates the only write to the err
variable in setup_rt_sigframe.

Always copy the siginfo structure.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211221025012.1057923-3-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>