Create and record the rt signal trampoline.

Reviewed-by: Stafford Horne <shorne@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210929130553.121567-18-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>