linux-user/s390x: Mask si_addr for SIGSEGV

linux-user/s390x: Mask si_addr for SIGSEGV

For s390x, the address passed to a signal handler in the
siginfo_t si_addr field is masked (in the kernel this is done in
do_sigbus() and do_sigsegv() in arch/s390/mm/fault.c). Implement
this architecture-specific oddity in linux-user.

This is one of the issues described in
https://bugs.launchpad.net/qemu/+bug/1705118

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>