Add -Zmiri-trace-ffi to emit a message per FFI call

[oli-obk]

Comparing miri and real evaluations can sometimes be hard due to the verbose logging of miri vs the difficulty of getting any information out of a real execution without changing its behaviour.

We could add a new flag that dumps the name and arguments of all shim invocations.

[RalfJung]

We don't know the exact syscalls that correspond to each of our library shims, and I don't think that's information we want to add to Miri.

[oli-obk]

I'd be fine with this just happening on all shims. Tho reducing it to known syscallish candidates would be less noisy

[RalfJung]

Well it's just a fundamentally different layer of abstraction -- the shims correspond to C library calls, which in the real system are implemented using between 0 and N syscalls.

[tiif]

I propose adding the return value to this like what strace did. This will help in following the trace.

For example in this trace:

fcntl(9, F_DUPFD_CLOEXEC, 3)            = 11
epoll_ctl(11, EPOLL_CTL_ADD, 12, {events=EPOLLIN|EPOLLRDHUP|EPOLLET, data={u32=1, u64=1}}) = 0

fcntl returns the value of new file descriptor, and it is easy to see that this file descriptor is passed to epoll_ctl.

[RalfJung]

Whatever we do, we should be careful not to require patching every single shim. That will be too tricky to keep consistent. Ideally, this is done in a single place.

I long wanted to reorganize shim dispatch to be more declarative; that may help here as well.