linux-user: Fix s390x safe-syscall for z900

The LT instruction was added in the extended immediate facility
introduced with the z9-109 processor.

Cc: Riku Voipio <riku.voipio@iki.fi>
Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Fixes: c9bc3437
Suggested-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>