seccomp: add "seccomp" syscall

This adds the new "seccomp" syscall with both an "operation" and "flags"
parameter for future expansion. The third argument is a pointer value,
used with the SECCOMP_SET_MODE_FILTER operation. Currently, flags must
be 0. This is functionally equivalent to prctl(PR_SET_SECCOMP, ...).

In addition to the TSYNC flag later in this patch series, there is a
non-zero chance that this syscall could be used for configuring a fixed
argument area for seccomp-tracer-aware processes to pass syscall arguments
in the future. Hence, the use of "seccomp" not simply "seccomp_add_filter"
for this syscall. Additionally, this syscall uses operation, flags,
and user pointer for arguments because strictly passing arguments via
a user pointer would mean seccomp itself would be unable to trivially
filter the seccomp syscall itself.

And fixup of unistd32.h to truly enable sys_secomp.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Andy Lutomirski <luto@amacapital.net>
Git-commit: e985fd474debedb269fba27006eda50d0b6f07ef
Git-repo: https://www.codeaurora.org/cgit/external/gigabyte/kernel/msm


[rsiddoji@codeaurora.org: dropped x86 and arm64 related changes]
Change-Id: I95bea02382c52007d22e5e9dc563c7d055c2c83f
Signed-off-by: Ravi Kumar Siddojigari <rsiddoji@codeaurora.org>