Add a helper that can make a (traced) 6-arg syscall.

Mostly straightforward except

* We previously relied on traced syscalls being made from libc
  (i.e. outside the syscallbuf lib) when stepping to deliver signals.
  Now they won't be, so we have to explicitly recognize traced
  syscalls.  (This is arguably better for clarity.)

* It's imperative for debugging that users can see backtraces from
  traced (and untraced) syscalls.  So CFI directives had to be added
  to the new _raw_traced_syscall() helper, shamelessly cadged from
  hand-written glibc assembly.

Resolves #687.