asm-generic: add generic seccomp.h for secure computing mode 1

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

Those values (__NR_seccomp_*) are used solely in secure_computing()
to identify mode 1 system calls. If compat system calls have different
syscall numbers, asm/seccomp.h may override them.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>