arm64: add SIGSYS siginfo for compat task

Note: This patch is from v6 of Takahiro's proposed
"arm64: add seccomp support" patchset (leecam@google.com)

SIGSYS is primarily used in secure computing to notify tracer.
This patch allows signal handler on compat task to get correct information
with SA_SYSINFO specified when this signal is delivered.

Signed-off-by: AKASHI Takahiro <takahiro.akashi <at> linaro.org>