linux-user/alpha: Share code for TARGET_NR_sigaction

linux-user/alpha: Share code for TARGET_NR_sigaction

There's no longer a difference between the alpha code and
the generic code.

There is a type difference in target_old_sigaction.sa_flags,
which can be resolved with a very much smaller ifdef, which
allows us to finish sharing the target_sigaction definition.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210422230227.314751-7-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>