linux-user: Fix register used for 6th and 7th syscall argument on aarch64

linux-user: Fix register used for 6th and 7th syscall argument on aarch64

This unbreaks the testcase from

    http://lists.nongnu.org/archive/html/qemu-arm/2018-01/msg00514.html

Thanks to Laurent Vivier for spotting the 7th one.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Suggested-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <671eaa99f4e0bf3a58f76f9151f7cfa24662227f.1517565566.git.agx@sigxcpu.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>