linux-user: Move tswap_siginfo out of target code

linux-user: Move tswap_siginfo out of target code

Move tswap_siginfo from target code to handle_pending_signal. This will
allow some cleanups and having the siginfo ready to be used in gdbstub.

Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>
Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240309030901.1726211-3-gustavo.romero@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>