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.

This supports x86_64, i386, and ARM if the syscall architecture
matches the libseccomp architecture.  It also supports i386 compat
syscalls with a 64-bit libseccomp.  (That 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.

(NB: ARM, in theory, supports syscalls with seven arguments.  The
only syscall like this is sys_syscall, which is (AFAICT) only exists
because it's awkward to implement syscall(2) on OABI without it.
seccomp does not, and probably never will, support filtering on the
seventh argument of sys_syscall, and there's little point for us to
have a nasty special case just to be able to decode it..)

Signed-off-by: Andy Lutomirski <luto@amacapital.net>