arm64: add seccomp support

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

secure_computing() is called first in syscall_trace_enter() so that a system
call will be aborted quickly without doing succeeding syscall tracing,
contrary to other cases, if seccomp rules deny that system call.

On compat task, syscall numbers for system calls allowed in seccomp mode 1
are different from those on normal tasks, and so _NR_seccomp_xxx_32's need
to be redefined.

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

Conflicts:
	arch/arm64/Kconfig
	arch/arm64/kernel/entry.S

Change-Id: I5ec44507d7e536df7ec9d62d30a418c26ef15100