Add seccomp_sigsys_decode

This will decode a SIGSYS to a struct seccomp_data.  It shows the raw
(untranslated) values as opposed to the translated values.  For example,
socketcall will show up as socketcall.

So far, this supports x86_64 native and i386 on x86_64.  (The latter
is unlikely to happen without either some kind of thunk library to load
32-bit code into a 64-bit process or explicit invocations of compat syscalls
from 64-bit code.)  It should also work on x32, although I haven't tested
it.

On ARM, seccomp_sigsys_decode will return -EOPNOTSUPP.  That means that
the new test will fail.  It should be very easy to add ARM support (it
should be modeled after the i386 code), but I'm not set up to be able
to test on ARM.

(NB: ARM, in theory, supports syscalls with seven arguments.  AFAICS there are
no such syscalls, and the kernel seccomp code wouldn't really support them
anyway.)

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Add ARM syscall decoding