add seccomp syscall for compat task

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

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

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

Conflicts:
	arch/arm64/include/asm/unistd32.h

Change-Id: I63d38f68da72b3333327256b4cacba2c3ddb39fc