Add an `fn_ptr` field to `MiriFrame`

Add an `fn_ptr` field to `MiriFrame`

The `backtrace-rs` crate can use this to implement
`Frame::symbol_address`, which is used to skip frames
above the call to `Backtrace::capture` on the stack.

The function pointer will not be useable for comparison purposes if the
function is generic, as CTFE creates a new function pointer for each
cast of a (monomorphized) generic function. However, this already
affects code running under Miri, and isn't a problem for `backtrace-rs`
(which only casts a non-generic function).

I've added logic to allow `MiriFrame` to have either 4 or 5 fields - if
a 5th field is present, we write the function pointer to it.