Do not directly access ucontext_t as the third signal parameter.
This is preparation for a sparc64 fix.
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
9940799bde