arm64: add seccomp syscall for compat task

This patch allows compat task to issue seccomp() system call.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
(cherry picked from commit e185fab7e13087cbd1f7e281d2698f07dbcdd44e)
Signed-off-by: Daniel Kurtz <djkurtz@google.com>