The `unwinding` crate supports processing unwinding data, and is written
entirely in Rust. This allows it to be ported to new platforms more
easily than using the llvm-based `libunwind`.

While `libunwind` is very well supported on major targets, it is
difficult to use on other targets. SGX is an example of this where Rust
carries custom patches in order to enable backtrace support.

This adds an alternative for supported architectures. Rather than
providing a custom target, `unwinding` allows for a solution that is
completely written in Rust.

This adds `xous` as the first consumer, and forthcoming patches will
modify libstd to take advantage of this.

Signed-off-by: Sean Cross <sean@xobs.io>

Now that `unwind` supports Xous, enable unwinding panics on Xous.

Signed-off-by: Sean Cross <sean@xobs.io>

Xous as an operating system is compiled with gcc-type personalities when
it comes to unwinding. This enables unwinding inside panics on Xous,
which enables Rust tests.

Signed-off-by: Sean Cross <sean@xobs.io>

Now that everything is in place to support unwinding on Xous, enable
this for that target.

Signed-off-by: Sean Cross <sean@xobs.io>

Add `unwinding` as a dependency. This is required on platforms where
unwinding isn't provided by llvm.

Signed-off-by: Sean Cross <sean@xobs.io>

The main() function takes an argument that contains the eh_frame
address. Implement `unwinding` support by looking for unwinding data at
this address.

Signed-off-by: Sean Cross <sean@xobs.io>

Add `unwinding` as a permitted dependency of rustc, as it is now used as
part of panic unwinding within platforms such as Xous.

Signed-off-by: Sean Cross <sean@xobs.io>