linux-user: Pass ka_restorer to do_sigaction

The value of ka_restorer needs to be saved in sigact_table.
At the moment, the attempt to save it in do_syscall is
improperly clobbering user memory.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210422230227.314751-4-richard.henderson@linaro.org>
[lv: remove tab]
Signed-off-by: Laurent Vivier <laurent@vivier.eu>