linux-user: Handle rt_sigaction correctly for SPARC

linux-user: Handle rt_sigaction correctly for SPARC

SPARC is like Alpha in its handling of the rt_sigaction syscall:
it takes an extra parameter 'restorer' which needs to be copied
into the sa_restorer field of the sigaction struct. The order
of the arguments differs slightly between SPARC and Alpha but
the implementation is otherwise the same. (Compare the
rt_sigaction() functions in arch/sparc/kernel/sys_sparc_64.c
and arch/alpha/kernel/signal.c.)

Note that this change is somewhat moot until SPARC acquires
support for actually delivering RT signals.

Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>